Passage Summary: A guide thinks the most common fish in a dirty pond is the one that handles pollution best. Scientists found out the bullhead is the most common fish. Therefore, the guide must think the bullhead is the best at handling pollution.

Conclusion: The fishing guide must believe that the bullhead catfish is the species in the pond that has adapted best to the pollution.

Reasoning: The guide believes the most populous fish is the best adapted, and recent studies have identified the bullhead catfish as the most populous species.

Analysis: This argument suffers from a 'knowledge' flaw, assuming that because a general principle is held by someone and a specific fact is true, that person must know the specific fact. The guide believes 'Most Populous = Best Adapted,' but does the guide know the bullhead is the most populous? For all we know, the guide mistakenly thinks the perch is the most populous and therefore believes the perch is the best adapted. Look for an answer that points out the guide might be unaware of the recent studies' findings.