The sentences that should replace "who" with "whom" to reflect correct usage are:
-
The water from the faucet has turned brown, but I'm not sure whom to call about it.
- In this sentence, "whom" is correct because it is the object of the preposition "to."
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Who do you think should be the next president of the science club? (no change necessary in this sentence).
- This one remains as "who" because it is the subject of the verb "should be" within the clause.
The other sentences involve "who" correctly as they serve as subjects in their respective clauses.
Thus, the correct answers where "who" should be replaced by "whom" are:
- The water from the faucet has turned brown, but I'm not sure whom to call about it.
- Dorothy is a professional pastry chef whom you can ask for advice.
- In the example with Dorothy, "whom" would be appropriate in a different phrasing about asking her advice, but your examples did not require alteration.
In summary, focusing solely on the provided examples, you would only alter the first sentence. The other examples would retain "who."