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 CALENDAR 
Clue: Civilization that produced the first known calendar with 365 days

Answer: ancient egypt


Question: Category: "FIRST" PHRASES 
Clue: The earliest known use of this term was in an Indianapolis Star opinion piece of September 20, 1914

Answer: first world war


Question: Category: VITAMINS & MINERALS 
Clue: The Centers for Disease Control & Prevention says it's the nutrient Americans are most likely to be deficient in

Answer:
iron