Passage Summary: A town banned certain soaps, and even though some people cheated by shopping out of town, the water pollution went down, leading the author to believe at least some people followed the new rules.

Conclusion: Some local residents must have switched to phosphate-free dishwasher detergents.

Reasoning: Despite reports of residents buying banned detergents elsewhere, the total phosphate pollution from the local treatment plant has dropped significantly.

Analysis: To evaluate this argument, we need to determine if the drop in pollution was actually caused by the residents' behavior or something else entirely. In the world of municipal waste, many things can affect pollution levels besides household soap. Look for an answer that asks about other potential sources of phosphate or changes in how the waste is processed. If a major industrial polluter also stopped using phosphates at the same time, the author's conclusion about the residents would be much weaker.