Blandenburg's population of flutterfish declined in the past decade

OFFICIAL EXPLANATION

This is a question in which our answer won't be directly included in the information the passage provides, so our best bet is to go through the answer choices and eliminate options that don't apply. But first, let's identify the logic of the argument Flutterfish disappeared because of factories. Hamrats also began to disappear because they eat the flutterfish that disappeared. Now, despite the fact that flutterfish are no longer being killed by factories, the hamrat population is still declining. With that in mind, let's check out the answers.
Choice (A) is incorrect because the question mentions a lack of food as the primary reason for declines in the hamrat population. If hamrats found another food source, their population would stabilize.
Choice (B) is incorrect because none of the information we have suggests that the presence of other fish would affect the hamrat population. Choice (C) could be correct, so let's put that aside and return to it later. Choice (D) is incorrect because the factories shut down and stopped killing flutterfish—the reason they shut down doesn't matter.
Choice (E) is incorrect because there's nothing that indicates conditions in other towns are related to conditions in Blandenburg.
It looks like (C) is correct. If factories altered the flow of the river, shutting them down wouldn't return it to its original course. If the river doesn't return to its original course, flutterfish will continue to spawn in their new location and may not return to Blandenburg. If the flutterfish don't return, hamrats don't regain their food source and the hamrat population will continue to decline.

The correct answer is Choice (C).