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No, the answer is not A. The statement about the clerk laughing is not a conclusion but rather a response to the author's request for a room with running water.
The best-supported conclusion is B. The text discusses the historical dangers of untreated water and the advancements in plumbing and sanitation that have made drinking water safe today. The mention of laws to protect our water supply suggests that we should be grateful for the improvements that have been made.