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?
A: 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.
B: During spawning season, the dam releases sufficient water for migratory fish below the dam to swim upstream.
C: Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years.
D: The construction of the dam stirred up potentially toxic river sediments that were carried downstream.
Choose A, B, C or D as your solution.
D