Below are answer explanations to the Science Test of the 4th released full-length Digital ACT practice test.

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Answer Explanations to the 4th Digital ACT Practice Science Test

Passage I – “Heavy Metals in Maize and Cowpea Plants”

Question 1, “A student claimed that the heavy metal with the smallest atomic number would have the lowest average concentration in cowpea plants grown in unpolluted soil. Is this claim supported by the data in Table 1?” The correct answer is C: “No; chromium has the smallest atomic number, but cadmium has the lowest concentration in cowpea plants grown in unpolluted soil.”

This question requires comparing the atomic numbers of the five metals with their concentrations in Table 1. Chromium has the smallest atomic number, 24, but cadmium has the lowest concentration in cowpea plants grown in unpolluted soil, at 3.12 ppm. Therefore, the student’s claim is not supported, making Choice C correct. Choice A incorrectly identifies chromium as having the lowest concentration. Choice B incorrectly identifies cadmium as having the smallest atomic number. Choice D correctly rejects the claim but reverses the metals’ characteristics, incorrectly identifying cadmium as having the smallest atomic number and chromium as having the lowest concentration.

Question 2, “Based on Table 1, for which of the heavy metals was the average concentration higher in cowpea plants grown in polluted soil than in maize plants grown in polluted soil?” The correct answer is G: “Chromium and copper only.”

This question requires comparing the concentrations of five metals in maize and cowpea plants grown in polluted soil. Cowpea plants contained 32.9 ppm of chromium and 15.30 ppm of copper, compared with 29.9 ppm and 9.24 ppm, respectively, in maize plants. These are the only two metals with higher concentrations in cowpea plants, making Choice G correct. Choice F omits copper. Choice H incorrectly identifies lead and cadmium, both of which had higher concentrations in maize plants. Choice J also incorrectly includes lead and cadmium while identifying nickel, which had equal concentrations of 39.1 ppm in both plants.

Question 3, “Based on Table 2, which combination of plant and soil type resulted in the smallest difference between the average plant height at the end of Week 3 and the average plant height at the end of Week 4?” The correct answer is B: “Plant: maize; Soil type: polluted.”

This question requires calculating the change in average plant height between Weeks 3 and 4 for each combination. Maize grown in unpolluted soil increased from 37.0 to 44.0 cm, a difference of 7.0 cm. Maize grown in polluted soil increased from 20.0 to 24.0 cm, a difference of 4.0 cm. Cowpea plants grown in unpolluted soil increased by 5.0 cm, while cowpea plants grown in polluted soil increased by 4.3 cm. The smallest difference is 4.0 cm for maize in polluted soil, making Choice B correct. Choices A, C, and D all correspond to larger increases.

Question 4, “Consider the kind of plant and soil type for which the average concentration of nickel at the end of 6 weeks of growth was 22.8 ppm. What was the average plant height for those plants at the end of Week 2?” The correct answer is H: “16.0 cm.”

This question requires using information from two tables. Table 1 shows that cowpea plants grown in unpolluted soil had a nickel concentration of 22.8 ppm. Once this combination is identified, Table 2 shows that these plants had an average height of 16.0 cm at the end of Week 2, making Choice H correct. Choice F (13.0 cm) corresponds to cowpea plants grown in polluted soil. Choice G (15.0 cm) corresponds to maize grown in polluted soil, while Choice J (33.0 cm) corresponds to maize grown in unpolluted soil. The correct approach is to identify the plant and soil conditions before locating the corresponding height.

Question 5, “Suppose the greater the total average concentration of heavy metals in a plant grown in polluted soil, the more effective the plant would have been at mitigating soil pollution. Based on Table 1, which of maize plants or cowpea plants would likely have been more effective at mitigating oil-polluted soil?” The correct answer is A: “Maize; the total average concentration of heavy metals in maize plants grown in polluted soil was greater than that of cowpea plants grown in polluted soil.”

This question requires adding the concentrations of all five heavy metals for each plant grown in polluted soil. For maize, the total is 35.50 + 9.05 + 29.9 + 39.1 + 9.24 = 122.79 ppm. For cowpea plants, the total is 8.40 + 6.68 + 32.9 + 39.1 + 15.30 = 102.38 ppm. Since maize has the greater total concentration, it would be more effective according to the question’s assumption, making Choice A correct. Choice B identifies the correct plant but incorrectly states that its total concentration was lower. Choices C and D incorrectly identify cowpea plants as more effective.

Passage II – “Measuring Ground-Level Ozone”

Question 6, “Based on Figures 1 and 3, the air sample collected at which of the following combinations of time and location had the lowest O₃ concentration?” The correct answer is G: “Time: 2:30 p.m.; Location: Site C.”

This question requires comparing the triiodide ion concentrations in Figure 3 and applying the relationship shown in Figure 1. Figure 1 establishes that higher I₃⁻ concentrations correspond to higher ozone concentrations. Among the four combinations, Site C at 2:30 p.m. has the lowest I₃⁻ concentration, approximately 0.75 μmol/L, making Choice G correct. Choice F corresponds to Site B at 2:30 p.m., when the concentration was approximately 2.2 μmol/L. Choice H identifies Site B at 4:00 p.m., when the concentration was approximately 2.25 μmol/L. Choice J identifies Site C at 4:00 p.m., when the concentration was approximately 1.4 μmol/L. All three indicate higher ozone concentrations.

Question 7, “Based on Figures 1 and 3, which site had the highest overall average O₃ concentration across all the air samples?” The correct answer is B: “Site B.”

This question requires comparing the overall patterns for the four sites in Figure 3. Because Figure 1 shows a direct positive relationship between I₃⁻ and O₃ concentrations, the site with the highest average I₃⁻ concentration also has the highest average ozone concentration. Site B consistently records concentrations near or above 2.0 μmol/L, generally higher than the other sites, making Choice B correct. Site A has somewhat lower concentrations overall, while Site C remains around 1.0–1.5 μmol/L. Site D has the lowest concentrations, generally near 0.5–0.7 μmol/L. Therefore, Choices A, C, and D do not represent the highest overall average.

Question 8, “Based on Figures 1 and 3, the O₃ concentration at 5:00 p.m. at Site B was approximately how many times as great as the O₃ concentration at 2:00 p.m. at Site C?” The correct answer is G: “2.”

This question requires comparing two measurements using the relationship between I₃⁻ and O₃ concentrations. Figure 3 shows that Site B had an I₃⁻ concentration of approximately 2.0 μmol/L at 5:00 p.m., while Site C had approximately 1.0 μmol/L at 2:00 p.m. Figure 1 indicates that these values correspond to ozone concentrations of approximately 80 ppb and 40 ppb, respectively. Dividing 80 by 40 gives 2, making Choice G correct. Choice F (0.5) reverses the ratio, while Choices H (3) and J (4) overestimate the difference between the two concentrations.

Question 9, “When a vacuum pump was operating during Step 2, it drew air through the solution in the test tube. Assuming that the pump drew air at a rate of 200 mL/min, how many milliliters of air were drawn through the solution in the test tube each time Step 2 was performed?” The correct answer is D: “5,000 mL.”

This question requires multiplying the rate of airflow by the duration of sample collection. Step 2 states that the vacuum pump operated for 25 minutes. At a rate of 200 mL/min, the total volume of air collected would be 200 × 25 = 5,000 mL, making Choice D correct. Choice A (200 mL) represents only one minute of airflow. Choice B (550 mL) does not correctly account for the stated rate and duration. Choice C (2,500 mL) represents only half the calculated volume. Multiplying the rate by the complete operating time gives the correct total.

Question 10, “At Site A, was the O₃ concentration highest at 4:00 p.m. or at 5:30 p.m.?” The correct answer is H: “5:30 p.m.; the I₃⁻ concentration was highest at 5:30 p.m., and the higher the I₃⁻ concentration, the higher the O₃ concentration.”

This question requires interpreting the relationship in Figure 1 and comparing two points in Figure 3. At Site A, the I₃⁻ concentration was approximately 1.55 μmol/L at 4:00 p.m. and approximately 2.4 μmol/L at 5:30 p.m. Because ozone concentration increases as I₃⁻ concentration increases, the ozone concentration was higher at 5:30 p.m., making Choice H correct. Choices F and G incorrectly identify 4:00 p.m. as having the higher ozone concentration. Choice J identifies the correct time but incorrectly suggests that lower I₃⁻ concentrations correspond to higher ozone concentrations, contradicting Figure 1.

Question 11, “Suppose that the actual O₃ concentration at 12:30 p.m. at Site C was 43 ppb. Based on Figures 1 and 3, which of the following expressions would give the percent error for the value of the O₃ concentration that was determined at 12:30 p.m. at Site C?” The correct answer is A: “(|40 ppb − 43 ppb| ÷ 43 ppb) × 100%.”

This question tests the calculation of percent error. Figure 3 shows an I₃⁻ concentration of approximately 1.0 μmol/L for Site C at 12:30 p.m., corresponding to an estimated ozone concentration of 40 ppb in Figure 1. The question provides an actual concentration of 43 ppb. Percent error is calculated by dividing the absolute difference between the measured and actual values by the actual value and multiplying by 100. Therefore, the correct expression is |40 − 43|/43 × 100%, making Choice A correct. Choice B incorrectly uses the measured value, 40 ppb, as the denominator. Choices C and D incorrectly use 100 ppb instead of the measured concentration.

Passage III – “Polyphenol Oxidase Activity”

Question 12, “Consider the PPO activities listed in Table 1 for Polyphenols R, S, and T at a pH of 7.5. What is the order of those polyphenols, from the polyphenol that resulted in the lowest PPO activity to the polyphenol that resulted in the highest PPO activity?” The correct answer is H: “Polyphenol S, Polyphenol R, Polyphenol T.”

This question requires comparing the PPO activities of three polyphenols at a specific pH. In Table 1, at a pH of 7.5, Polyphenol S has an activity of 3.2 mmol/min, Polyphenol R has an activity of 4.1 mmol/min, and Polyphenol T has an activity of 16.5 mmol/min. Arranging these values from lowest to highest gives S, R, T, making Choice H correct. Choices F and G incorrectly place Polyphenol R before Polyphenol S, despite R having a higher activity. Choice J correctly begins with S but incorrectly places T before R, reversing the order of the remaining two polyphenols.

Question 13, “Consider the PPO activities listed in Table 2 for Polyphenol Q and for Polyphenol T. For which of these polyphenols, if either, did the PPO activity always increase as temperature increased from 10°C through 35°C?” The correct answer is B: “Polyphenol T only.”

This question requires identifying whether PPO activity increases consistently across all listed temperatures. Table 2 shows that Polyphenol T’s activity rises from 6.8 mmol/min at 10°C to 11.5 mmol/min at 35°C, increasing at every recorded temperature. Polyphenol Q, however, increases from 11.5 mmol/min at 10°C to 18.0 mmol/min at 25°C before decreasing to 15.5 and then 14.2 mmol/min. Therefore, only Polyphenol T increases throughout the entire range, making Choice B correct. Choice A incorrectly selects Q, Choice C incorrectly includes both polyphenols, and Choice D overlooks the consistent increase shown for T.

Question 14, “Based on the passage, the chemical reaction catalyzed by PPO is best represented by which of the following expressions?” The correct answer is J: “polyphenol + O₂ → o-quinone.”

This question tests the ability to identify reactants and products in a chemical reaction. The passage explains that PPO catalyzes a reaction between polyphenols and oxygen, producing o-quinones. Therefore, polyphenol and O₂ must appear on the left side of the equation, with o-quinone on the right, making Choice J correct. Choices F and G incorrectly identify o-quinone as a reactant rather than the product. Choice H correctly places polyphenol on the reactant side but incorrectly identifies oxygen as a product. Choice J is the only expression consistent with the reaction described in the passage.

Question 15, “Based on Table 1, for Polyphenol S, how many millimoles of the product of the reaction catalyzed by PPO would be formed in 5 min if the solution had a pH of 7.0?” The correct answer is B: “20 mmol.”

This question requires using the reaction rate from Table 1 to calculate the quantity of product formed over time. At a pH of 7.0, the PPO activity for Polyphenol S is 4.0 mmol/min. Over five minutes, the total amount of product formed would be 4.0 × 5 = 20 mmol, making Choice B correct. Choice A (15 mmol) and Choice C (23 mmol) do not match the calculation using the specified reaction rate. Choice D (26 mmol) results from using the activity of Polyphenol R at pH 7.0, 5.2 mmol/min, rather than the activity of Polyphenol S.

Question 16, “How many of the pH values listed in Table 1 are classified as basic?” The correct answer is F: “1.”

This question tests knowledge of the pH scale. A solution is classified as basic when its pH is greater than 7. Table 1 lists pH values of 5.0, 5.5, 6.0, 6.5, 7.0, and 7.5. Only 7.5 is greater than 7, so there is one basic pH value, making Choice F correct. Choice G (2) may result from incorrectly including pH 7.0, which is neutral. Choices H (3) and J (4) would require counting additional acidic or neutral values as basic. Choice F correctly identifies the only basic solution in the table.

Passage IV – “The Transit of Venus”

Question 17, “Suppose the group at Location 1 had recorded the time they observed Venus in the first contact position. This time, in UTC, would most likely have been:” The correct answer is A: “before 05:34:05.”

This question requires understanding the sequence of contact positions shown in Figure 1. First contact occurs when Venus initially touches the edge of the Sun’s disk, before it moves completely inside the disk at second contact. Table 1 shows that Location 1 observed second contact at 05:34:05 UTC. Therefore, first contact must have occurred before that time, making Choice A correct. Choices B and C identify times after second contact had already occurred, while Choice D identifies a time after third contact. Since first contact is the earliest position in the sequence, none of these later times is possible.

Question 18, “The term ‘interior egress’ refers to the point during a transit at which the planet is seen near the end of its transit, is still seen entirely within the disk, and is just touching the inside edge of the disk. Which contact position described in Figure 1 corresponds to the interior egress?” The correct answer is H: “Third contact.”

This question requires matching a scientific description to the labeled positions in Figure 1. Third contact occurs when Venus is still completely inside the Sun’s disk but has reached the edge and is about to begin leaving it. This matches the definition of interior egress, making Choice H correct. Choice F (first contact) describes Venus initially touching the outside edge of the solar disk. Choice G (second contact) occurs when Venus has just moved completely inside the disk near the beginning of the transit. Choice J (fourth contact) occurs after Venus has completely exited the disk. Only third contact matches the specified position near the end of the transit.

Question 19, “Based on Table 1, as observed from any of Locations 1–6, the amount of time it took for Venus to move from second contact to third contact was closest to which of the following?” The correct answer is A: “5 hr 25 min.”

This question requires calculating the elapsed time between second and third contact. For example, at Location 1, second contact occurred at 05:34:05 UTC, and third contact occurred at 10:59:30 UTC. Subtracting gives an elapsed time of 5 hours, 25 minutes, and 25 seconds, approximately 5 hours and 25 minutes. Choice A is therefore correct. Choice B (5 hr 40 min) is longer than the observed interval, while Choice C (11 hr 5 min) is approximately the time of third contact rather than the elapsed time. Choice D (16 hr 40 min) greatly exceeds the duration indicated by the data.

Question 20, “The international date line is located about 180° east from the prime meridian (0° longitude). Based on Table 1, which location was closest to the international date line?” The correct answer is F: “Location 1.”

This question requires comparing the longitudes in Table 1 with the approximate longitude of the international date line, 180°. Location 1 is at 104.4°E, placing it approximately 75.6° from the date line. Location 2 is at 51.4°E, while Locations 3 and 4 are at 22.0°W and 4.3°W, respectively. All three are farther from 180° longitude than Location 1. Therefore, Choice F is correct. Choices G, H, and J correspond to locations closer to the prime meridian and farther from the international date line.

Question 21, “A student predicted that the farther west the location, the later the time that second contact would be observed. Do the results for Locations 2, 3, and 4 support this prediction?” The correct answer is D: “No, because Location 3 was farther west than both Locations 2 and 4, but the second contact was observed at Location 4 later than it was at either Location 2 or Location 3.”

This question requires comparing the longitudes and second-contact times of three locations. Location 3, at 22.0°W, is farther west than Location 4 at 4.3°W and Location 2 at 51.4°E. However, Location 3 observed second contact at 05:38:47 UTC, while Location 4 observed it later, at 05:39:26 UTC. Therefore, moving farther west did not always correspond to a later observation time, making Choice D correct. Choice A incorrectly identifies Location 4 as the farthest west. Choice B correctly identifies Location 3 as the farthest west but incorrectly claims that it observed second contact last. Choice C also incorrectly identifies Location 4 as the farthest west.

Question 22, “Suppose that students at an additional location were able to record one, but not both, of the second and third contact times because sunset occurred at their location during the transit. Which of those contact positions would more likely not have been recorded?” The correct answer is J: “Third contact, because the solar disk would have been below the horizon.”

This question requires understanding the timing of the transit and the effect of sunset on observations. Second contact occurs earlier than third contact. If students observed one contact before sunset but could not observe the other because the Sun had set, the later event—third contact—would be the one missed. After sunset, the Sun would be below the horizon, preventing observation. Choice J correctly identifies both the missed contact and the reason. Choices F and G incorrectly identify second contact as the missed event. Choice H correctly identifies third contact but incorrectly states that the Sun would have been above the horizon.

Passage V – “Terminal Velocity of Air Bubbles”

Question 23, “Which of the following statements describes a difference between Study 2 and Study 3? In Study 2:” The correct answer is D: “the liquid in the GC was 10 mM KCl, whereas in Study 3, the liquid in the GC was 100 mM KCl.”

This question tests the ability to identify differences in experimental procedures. Study 2 used a 10 mM potassium chloride (KCl) solution, while Study 3 used a 100 mM KCl solution. Choice D correctly identifies this difference. Choice A incorrectly suggests that the studies tested different ranges of platform angles, although both used the same seven angles. Choice B incorrectly claims that the studies used different numbers of bubble radii, despite following the same procedures. Choice C incorrectly states that Study 2 used water and Study 3 used 10 mM KCl. Those conditions correspond to Studies 1 and 2, respectively, rather than Studies 2 and 3.

Question 24, “Based on the results of the studies, for any particular liquid at any particular platform angle, as bubble radius increased, terminal velocity:” The correct answer is F: “increased only.”

This question requires identifying the relationship between bubble radius and terminal velocity in Figures 2, 3, and 4. In each figure, every curve rises as bubble radius increases, indicating that larger bubbles consistently have higher terminal velocities at a given platform angle and liquid composition. Choice F correctly describes this pattern. Choice G incorrectly predicts that terminal velocity decreases as radius increases. Choice H suggests that terminal velocity initially increases and then decreases, while Choice J suggests the opposite pattern. Neither occurs in the plotted data. The consistent upward trend across all three figures supports Choice F.

Question 25, “Consider the radii of the bubbles released into the GC at θ = 4.2° during Studies 1 and 3. In which study was the bubble having the greatest radius observed, and approximately what was that radius?” The correct answer is C: “Study 3 and 1.27 mm.”

This question requires comparing the curves corresponding to a platform angle of 4.2° in Figures 2 and 4. According to the keys, these curves are represented by open squares. In Figure 2, representing Study 1, the largest plotted bubble radius at this angle is approximately 1.1 mm. In Figure 4, representing Study 3, the largest plotted radius is approximately 1.27 mm. Therefore, the largest bubble was observed in Study 3, making Choice C correct. Choices A and B incorrectly identify Study 1, while Choice D identifies the correct study but overestimates the radius as 1.57 mm.

Question 26, “Suppose that during Study 1, the platform had been set to an angle of 7.3°. A bubble with a terminal velocity of 30 mm/s would most likely have had a radius:” The correct answer is H: “between 0.40 mm and 0.88 mm.”

This question requires estimating a value between two measured platform angles. Figure 2 shows that an angle of 7.3° falls between 4.2° and 10.4°. At a terminal velocity of 30 mm/s, the curve for 10.4° corresponds to a bubble radius of approximately 0.4 mm, while the curve for 4.2° corresponds to approximately 0.88 mm. A bubble at the intermediate angle would therefore most likely have a radius between these values, making Choice H correct. Choices F and G predict radii below the range suggested by the neighboring curves, while Choice J predicts a radius greater than the value associated with the smaller platform angle.

Question 27, “What was the dependent variable in Study 1?” The correct answer is D: “Terminal velocity.”

This question tests identification of experimental variables. The dependent variable is the quantity measured in response to changes in the independent variables. In Study 1, researchers changed the platform angle and examined bubbles with different radii, then measured their terminal velocities. Therefore, terminal velocity is the dependent variable, making Choice D correct. Choice A (bubble radius) and Choice B (platform angle) identify variables that were deliberately varied during the experiment. Choice C (liquid composition) was held constant in Study 1 because the glass cylinder contained water throughout the study. The dependent variable is the measured outcome, not a condition that researchers controlled.

Question 28, “Based on the results of Study 3, the terminal velocity at θ = 10.4° for a bubble having a radius of 0.13 mm would have been approximately the same as the terminal velocity at θ = 2.6° for a bubble having which of the following radii?” The correct answer is G: “0.77 mm.”

This question requires comparing two curves in Figure 4. The curve for θ = 10.4°, represented by stars, indicates a terminal velocity of approximately 10 mm/s for a bubble with a radius of 0.13 mm. Following that velocity horizontally to the θ = 2.6° curve, represented by filled circles, gives a bubble radius of approximately 0.77 mm. Choice G is therefore correct. Choice F (0.60 mm) corresponds to a lower terminal velocity on the 2.6° curve. Choices H (1.06 mm) and J (1.50 mm) correspond to considerably higher terminal velocities. Choice G most closely matches the required speed.

Passage VI – “Nuclear Fusion Reactions”

Question 29, “Students 3 and 4 would agree on which of the following statements regarding either Reaction 1 or Reaction 2?” The correct answer is B: “Reaction 1 cannot occur.”

This question requires comparing the conclusions of Students 3 and 4. Student 3 argues that Reaction 1 cannot occur because it produces only one product, while a valid fusion reaction must produce at least two products according to that student’s criteria. Student 4 also concludes that Reaction 1 cannot occur, although this student rejects both proposed reactions. Therefore, the two students agree that Reaction 1 is impossible, making Choice B correct. Choice A directly contradicts both students. Choice C reflects Student 3’s conclusion about Reaction 2 but not Student 4’s. Choice D reflects Student 4’s conclusion about Reaction 2 but not Student 3’s.

Question 30, “Based on Student 1’s description of conservation, which of the following equations best represents the conservation of mass number for Reaction 2?” The correct answer is J: “2 + 3 = 4 + 1.”

This question requires distinguishing mass numbers from atomic numbers. In Reaction 2, the reactants are hydrogen nuclei with mass numbers of 2 and 3. The products are a helium nucleus with a mass number of 4 and a neutron with a mass number of 1. Conservation of mass number therefore requires 2 + 3 = 4 + 1, making Choice J correct. Choice F (1 + 1 = 2) and Choice G (1 + 1 = 2 + 0) involve atomic numbers rather than mass numbers. Choice H (2 + 2 = 4) describes the mass-number relationship for Reaction 1 rather than Reaction 2.

Question 31, “Of the 4 reactions that the students were shown, which 2 have the same set of reactants?” The correct answer is B: “Reaction 1 and Reaction Z.”

This question requires comparing the reactants on the left side of all four nuclear equations. Reaction 1 and Reaction Z both begin with two hydrogen nuclei, each having a mass number of 2 and an atomic number of 1. Therefore, they have identical reactants, making Choice B correct. Reaction 2 uses hydrogen nuclei with mass numbers of 2 and 3, while Reaction Y uses two helium nuclei with mass numbers of 3. Choices A and C incorrectly pair reactions with different elements among their reactants. Choice D incorrectly pairs Reaction 2 with Reaction Z, even though their hydrogen isotopes differ.

Question 32, “A particular lithium (Li) nucleus contains 3 protons and 4 neutrons. Based on the available information, which of the following symbols best represents this Li nucleus?” The correct answer is G: “⁷₃Li.”

This question tests interpretation of nuclear notation. The mass number is the total number of protons and neutrons, while the atomic number is the number of protons. A lithium nucleus with three protons and four neutrons has a mass number of 3 + 4 = 7 and an atomic number of 3. Therefore, its symbol is ⁷₃Li, making Choice G correct. Choice F incorrectly uses 4 as the mass number. Choice H incorrectly uses 4 for both the mass and atomic numbers. Choice J correctly identifies the mass number as 7 but incorrectly uses 4 as the atomic number, confusing the neutron count with the proton count.

Question 33, “Assume that protons are neither created nor destroyed in the reactions mentioned in the passage. How many protons in total are present in Reaction Y?” The correct answer is B: “4.”

This question requires identifying the total number of protons in a nuclear reaction. Reaction Y begins with two helium nuclei, each having an atomic number of 2. Since atomic number represents the number of protons, the reactants contain 2 + 2 = 4 protons. The question states that protons are conserved, so the reaction contains four protons in total, making Choice B correct. Choice A (2) counts the protons in only one helium nucleus. Choice C (6) confuses the total mass number of the reactants with the proton count. Choice D (8) overcounts the protons rather than recognizing that the same four protons are conserved throughout the reaction.

Question 34, “Which of the following fusion reactions can occur according to Student 1 but cannot occur according to Student 2?” The correct answer is F: “²₁H + ²₁H → ³₂He + ¹₀n.”

This question requires applying the different criteria proposed by Students 1 and 2. Student 1 requires conservation of mass number and atomic number. In Choice F, the mass numbers are conserved because 2 + 2 = 3 + 1, and the atomic numbers are conserved because 1 + 1 = 2 + 0. Therefore, Student 1 would accept this reaction. Student 2, however, requires every product to be positively charged when all reactants are positively charged. Because the neutron has no charge, Student 2 would reject the reaction, making Choice F correct. Choices G and J fail to conserve mass number, so Student 1 would reject them. Choice H conserves the required quantities and produces only positively charged particles, so both students would accept it.

Passage VII – “Marine Snail Escape Behavior”

Question 35, “Based on the description of Table 1, average t represents the average time between:” The correct answer is C: “when observations began and the first G. gibberulus jump.”

This question requires identifying the meaning of a variable defined in the study’s procedures. Step 3 states that t represents the time between the beginning of observations and the first jump made by G. gibberulus. Table 1 reports the average of this measurement for each pair of groups, making Choice C correct. Choice A incorrectly describes the time between successive jumps by G. gibberulus. Choice B refers to jumps by C. marmoreus, which were not measured. Choice D also incorrectly identifies C. marmoreus as the jumping snail. The recorded time specifically concerns the first escape jump by G. gibberulus.

Question 36, “Which of the following was the most likely reason a barrier was placed in the tank?” The correct answer is F: “To prevent the successful catch of prey.”

This question requires interpreting the experimental setup. The researchers placed a thin, transparent barrier between the predator, C. marmoreus, and its prey, G. gibberulus. Because the barrier physically separated the snails while allowing them to face one another, it most likely prevented the predator from catching the prey during observations, making Choice F correct. Choice G is incorrect because the barrier was placed in the center of the tank rather than around its edges, so preventing escape from the tank was not its purpose. Choice H is incorrect because the barrier was transparent, allowing visual contact. Choice J is incorrect because each experimental tank contained only one seawater condition, so separating different water types was unnecessary.

Question 37, “In Step 2 of the study, was each Group 4 snail placed facing toward the barrier or away from the barrier, and was that snail a predator or a prey to the Group 2 snail?” The correct answer is A: “Toward the barrier; predator.”

This question requires combining information about the experimental groups and the positioning of the snails. The passage explains that Groups 3 and 4 contained C. marmoreus, the predatory snail, while Groups 1 and 2 contained G. gibberulus, its prey. Step 2 states that one snail of each species was placed on opposite sides of the barrier, facing each other. Therefore, each Group 4 snail was a predator facing toward the barrier and the Group 2 prey snail, making Choice A correct. Choice B incorrectly identifies the Group 4 snail as prey. Choice C correctly identifies it as a predator but incorrectly states that it faced away from the barrier. Choice D is incorrect on both counts.

Question 38, “Suppose the study had been repeated and the average t for one of the groups had been 2 min. Based on the results shown in Table 1, would the CO₂ concentration of seawater for that group more likely have been an ambient CO₂ concentration or an elevated CO₂ concentration?” The correct answer is J: “Elevated, because the average t would have been closer to that for Group 2.”

This question requires converting units and comparing the result with Table 1. An average t of 2 minutes equals 120 seconds. Table 1 shows that Group 1, which was kept in seawater with an ambient CO₂ concentration, had an average t of 58 seconds. Group 2, which was kept in seawater with elevated CO₂, had an average t of 118 seconds. Since 120 seconds is much closer to 118 seconds than to 58 seconds, elevated CO₂ is the more likely condition, making Choice J correct. Choices F and G incorrectly identify ambient CO₂, while Choice H identifies elevated CO₂ but incorrectly associates it with Group 1.

Question 39, “A scientist made 2 predictions: on average, the NJ for Group 1 would be greater than that for Group 2, and 80% of the snails in Group 1 would jump at least once. Are both of these predictions supported by the results of the study?” The correct answer is C: “No; on average, the snails in Group 1 jumped 3 additional times than did the snails in Group 2, but only 60% of the snails in Group 1 jumped at least once.”

This question requires evaluating two predictions using Table 1. The average number of jumps was 10 for Group 1 and 7 for Group 2, so Group 1 averaged three additional jumps, supporting the first prediction. However, the passage states that 80 G. gibberulus were divided equally into two groups, giving Group 1 a total of 40 snails. Table 1 shows that 24 jumped at least once, so the percentage was 24/40 × 100 = 60%, not 80%. Therefore, only the first prediction is supported, making Choice C correct. Choices A and B incorrectly accept both predictions. Choice D correctly rejects the second prediction but incorrectly states that Group 1 averaged eleven additional jumps.

Question 40, “The ambient ocean CO₂ concentration is approximately 415 parts per million (ppm). Consider the pair of groups for which the average value of d was 230 mm. Based on the description of the study and results in Table 1, snails in these groups were kept in seawater having a CO₂ concentration closest to which of the following?” The correct answer is J: “1,040 ppm.”

This question requires identifying the experimental condition and calculating the corresponding CO₂ concentration. Table 1 shows that Groups 2 and 4 had an average d of 230 mm. The study explains that these groups were kept in seawater with an elevated CO₂ concentration equal to 2.5 times the ambient concentration. Multiplying 415 ppm by 2.5 gives 1,037.5 ppm, which rounds to approximately 1,040 ppm, making Choice J correct. Choice F (415 ppm) represents the ambient concentration rather than the elevated concentration. Choices G (625 ppm) and H (830 ppm) underestimate the required concentration because they do not apply the specified multiplier of 2.5.

Additional ACT practice test answer explanations