You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For consistency, answers should be in all lower cased letters.
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Question: Category: THE 1930s 
Clue: On September 18, 1934 the Soviet Union joined this international organization

Answer: league of nations


Question: Category: SEX CHANGES 
Clue: Don't let the pearls fool you, these lady bivalves can change into males when it suits them

Answer: oysters


Question: Category: BOOK SEQUELS? 
Clue: Natty Bumppo discovers there's "The Penultimate Iroquois" in this man's 1820s sequel

Answer:
james fenimore cooper