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This is one of those cases where it is important to remember that possessive pronouns never take apostrophes, even though possessive nouns do (see it’s/its). “Who’s” always and forever means only “who is,” as in “Who’s that guy with the droopy mustache?” or “who has,” as in “Who’s been eating my porridge?” “Whose” is the possessive form of “who” and is used as follows: “Whose dirty socks are these on the breakfast table?”