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Khan Academy
Khan Academy: Advanced (Plural) Possession
Possessive plurals have a complicated relationship with apostrophes, but this exercise is here to help you understand it!
English Plus+
Grammar Slammer's Apostrophes Showing Possession
This site discusses the correct and incorrect usage of apostrophes to show possession and examples are provided. Note that links are provided at the bottom of the page for other specific uses of the apostrophe.