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  3. To prevent a newly built dam on the Chiff River from blocking the route of fish migrating to breeding grounds upstream, the dam

To prevent a newly built dam on the Chiff River from blocking the route of fish migrating to breeding grounds upstream, the dam

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问题 To prevent a newly built dam on the Chiff River from blocking the route of fish migrating to breeding grounds upstream, the dam includes a fish pass, a mechanism designed to allow fish through the dam. Before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season. But in the first season after the project’s completion, only 300 per day made the journey. Clearly, the fish pass is defective.
Which of the following, if true, most seriously weakens the argument?
选项 A、Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again.
B、On other rivers in the region, the construction of dams with fish passes has led to only small decreases in the number of fish migrating upstream.
C、The construction of the dam stirred up potentially toxic river sediments that were carried downstream.
D、Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years.
E、During spawning season, the dam releases sufficient water for migratory fish below the dam to swim upstream.
答案C
解析 Argument Evaluation
Situation A new dam includes a mechanism called a fish pass designed to allow fish to migrate upstream past the dam to their breeding grounds. The number of migrating fish fell from several thousand per day before the dam was built to three hundred per day in the first season after it was built, indicating—according to the argument—that the fish pass is defective.
Reasoning What evidence would suggest that the fish pass is not defective? The argument implicitly reasons that a defective fish pass would make it difficult for the fish to migrate, which would explain why the number of migrating fish fell when the dam was completed. Any evidence suggesting an alternative explanation for the reduced number of migrating fish, such as an environmental change that occurred when the dam was built, would cast doubt on the argument’s reasoning.
A A defective fish pass could prevent most of the fish from migrating upstream regardless of whether those that succeed ever return downstream.
B This would suggest that dams with properly functioning fish passes do not greatly reduce the number of migrating fish, so it would provide further evidence that the fish pass in this particular dam is defective.
C Correct. This suggests that the toxic sediments may have poisoned the fish and reduced their population. A smaller fish population could be sufficient to explain the reduced number offish migrating, which casts doubt on the argument’s assumption that the explanation for their declining numbers involves the fish pass.
D A slight and gradual ongoing decline in migratory fish populations would not explain an abrupt and extreme decline right after the dam was built.
E This supports the argument’s proposed explanation for the declining fish population by ruling out the alternative explanation that the dam does not release enough water for the fish to migrate.
The correct answer is C.
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