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" Speaking of whose, the one truly unforgivable sin that haunts the use of pronouns is the confusion of whose with who’s and its with it’s. Pronouns, when they get possessive, act weird. We do not say I’s, you’s, he’s, or she’s to indicate possession, so why would we write who’s or it’s? Possessive pronouns are all apostropheless: my, your, his, hers, its. Who’s and it’s are contractions of who is and it is (or who has and it has). Learn this or die. "
― Constance Hale , Sin and Syntax: How to Craft Wickedly Effective Prose