In the context of your sentences, here’s a breakdown of the correct usage of "who," "whom," "who's," and "whos":
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Who: This is used as a subject in a sentence. In your first sentence, "Who do you think will be attending the meeting?" is correct.
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Who's: This is a contraction for "who is" or "who has." Neither of your examples requires a contraction, but if you were saying "Who's attending the meeting?" that would be correct.
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Whom: This is used as an object in a sentence. It's uncommon in everyday conversation, but still technically correct. For example, "To whom should I address the letter?"
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Whos: This is a common misspelling and should not be used. The correct forms are "who," "whom," and "who's."
Based on your notes, if you're referring to someone (e.g., "Whom should I invite to the meeting?"), then "whom" would be appropriate. Otherwise, for general subjects or actions, you would use "who" or "who's."
If the correction is strictly about usage and context, ensure to replace "who's" with "who" if it’s in a subject position or keep it as "whom" in an object position as needed.