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14 Aug 2025, 08:44
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A research team discovered that a species of genetically modified (GM) salmon had escaped from a nearby aquaculture facility and interbred with wild salmon in a local river system. In the first spawning season after the escape, genetic testing of juvenile wild salmon confirmed the presence of GM genes. The facility was then shut down and all remaining GM salmon were removed. Three years later, a follow-up study that sampled hundreds of wild salmon found no trace of GM genes.
Which of the following, if true, most helps to explain the discrepancy between the two studies?
A. Offspring of genetically modified and wild salmon had significantly lower survival rates in the river environment than offspring of two wild salmon.
B. Natural predators tend to avoid genetically modified salmon because of differences in their taste.
C. Genetically modified salmon are more likely to escape into ocean environments than remain in rivers.
D. Wild salmon in the river system typically migrate to distant spawning grounds and return only every few years.
E. Fewer than ten adult GM salmon escaped before the facility was closed.
Which of the following, if true, most helps to explain the discrepancy between the two studies?
A. Offspring of genetically modified and wild salmon had significantly lower survival rates in the river environment than offspring of two wild salmon.
B. Natural predators tend to avoid genetically modified salmon because of differences in their taste.
C. Genetically modified salmon are more likely to escape into ocean environments than remain in rivers.
D. Wild salmon in the river system typically migrate to distant spawning grounds and return only every few years.
E. Fewer than ten adult GM salmon escaped before the facility was closed.
14 Aug 2025, 08:44
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Official Solution:
A research team discovered that a species of genetically modified (GM) salmon had escaped from a nearby aquaculture facility and interbred with wild salmon in a local river system. In the first spawning season after the escape, genetic testing of juvenile wild salmon confirmed the presence of GM genes. The facility was then shut down and all remaining GM salmon were removed. Three years later, a follow-up study that sampled hundreds of wild salmon found no trace of GM genes.
Which of the following, if true, most helps to explain the discrepancy between the two studies?
A. Offspring of genetically modified and wild salmon had significantly lower survival rates in the river environment than offspring of two wild salmon.
B. Natural predators tend to avoid genetically modified salmon because of differences in their taste.
C. Genetically modified salmon are more likely to escape into ocean environments than remain in rivers.
D. Wild salmon in the river system typically migrate to distant spawning grounds and return only every few years.
E. Fewer than ten adult GM salmon escaped before the facility was closed.
The conclusion of the passage-that genetically modified (GM) salmon genes disappeared from the wild population-stands in contrast to the earlier finding that those genes had entered wild salmon. To explain this discrepancy, we need a reason why the modified genes, once present, would no longer be detectable in the follow-up study.
(A) - Correct Answer. provides a compelling explanation: offspring resulting from interbreeding between GM and wild salmon had significantly lower survival rates than offspring of two wild salmon. If these hybrid offspring were less likely to survive and reproduce in the river environment, their genetic contribution would diminish over time. This natural selection effect would explain how the modified genes, although initially present, disappeared from the wild population by the time of the second study. This choice actually resolves the apparent contradiction in the passage.
(B) This strengthens the discrepancy. If GM salmon were more likely to survive due to predator avoidance, one would expect the genes to persist, not disappear.
(C) Does not explain the disappearance of the genes from the wild salmon in the river. The original study already confirmed the presence of those genes in the wild population, so movement of the GM salmon themselves to the ocean is irrelevant and this choice does not address why there are no offspring in the river system.
(D) Trap answer choice, as it only focuses on the Wild Salmon. This does not resolve the disappearance of modified genes from the population in the river environment.
(E) This answer choice is not helpful to explain the discrepancy. Even if a small number of GM salmon had escaped, they were detected in sampling originally, and we would expect that the sampling would increase or at least show some signs of impact. Not seeing any GM genes is not explained by this answer choice.
Answer: A
A research team discovered that a species of genetically modified (GM) salmon had escaped from a nearby aquaculture facility and interbred with wild salmon in a local river system. In the first spawning season after the escape, genetic testing of juvenile wild salmon confirmed the presence of GM genes. The facility was then shut down and all remaining GM salmon were removed. Three years later, a follow-up study that sampled hundreds of wild salmon found no trace of GM genes.
Which of the following, if true, most helps to explain the discrepancy between the two studies?
A. Offspring of genetically modified and wild salmon had significantly lower survival rates in the river environment than offspring of two wild salmon.
B. Natural predators tend to avoid genetically modified salmon because of differences in their taste.
C. Genetically modified salmon are more likely to escape into ocean environments than remain in rivers.
D. Wild salmon in the river system typically migrate to distant spawning grounds and return only every few years.
E. Fewer than ten adult GM salmon escaped before the facility was closed.
The conclusion of the passage-that genetically modified (GM) salmon genes disappeared from the wild population-stands in contrast to the earlier finding that those genes had entered wild salmon. To explain this discrepancy, we need a reason why the modified genes, once present, would no longer be detectable in the follow-up study.
(A) - Correct Answer. provides a compelling explanation: offspring resulting from interbreeding between GM and wild salmon had significantly lower survival rates than offspring of two wild salmon. If these hybrid offspring were less likely to survive and reproduce in the river environment, their genetic contribution would diminish over time. This natural selection effect would explain how the modified genes, although initially present, disappeared from the wild population by the time of the second study. This choice actually resolves the apparent contradiction in the passage.
(B) This strengthens the discrepancy. If GM salmon were more likely to survive due to predator avoidance, one would expect the genes to persist, not disappear.
(C) Does not explain the disappearance of the genes from the wild salmon in the river. The original study already confirmed the presence of those genes in the wild population, so movement of the GM salmon themselves to the ocean is irrelevant and this choice does not address why there are no offspring in the river system.
(D) Trap answer choice, as it only focuses on the Wild Salmon. This does not resolve the disappearance of modified genes from the population in the river environment.
(E) This answer choice is not helpful to explain the discrepancy. Even if a small number of GM salmon had escaped, they were detected in sampling originally, and we would expect that the sampling would increase or at least show some signs of impact. Not seeing any GM genes is not explained by this answer choice.
Answer: A
