The pronoun "who" should be replaced with "whom" in the following sentences:
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The water from the faucet has turned brown, but I'm not sure whom to call about it. (Because "whom" is the object of the verb "call.")
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Who do you think should be the next president of the science club? (The sentence is correct as it is, but if rephrased, "Who do you think is the next president of the science club?" would use "whom” in a structure like "Whom do you think is best suited...").
Thus, the relevant sentence for replacement would be:
- "The water from the faucet has turned brown, but I'm not sure whom to call about it."