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?
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AFish that have migrated to the upstream breeding grounds do not return down the Chiff River again.
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BOn 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.
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CThe construction of the dam stirred up potentially toxic river sediments that were carried downstream.
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DPopulations of migratory fish in the Chiff River have been declining slightly over the last 20 years.
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EDuring spawning season, the dam releases sufficient water for migratory fish below the dam to swim upstream
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