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 simplicity, answers should be in all lower cased letters.
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Question: Category: THE WHITE HOUSE
Clue: When electric lights were installed in 1891, this pres., afraid of being shocked, wouldn't operate the switches

Answer: benjamin harrison

Question: Category: U.S. RIVERS
Clue: The Falls of Saint Anthony stop this mighty river from being navigable north of St. Paul, Minnesota

Answer: the mississippi

Question: Category: MOVIES BY INITIALS
Clue: 2008, starring Robert Downey Jr.: "I.M."

Answer:
iron man