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: EUROPEAN PRINCESSES 
Clue: On July 20, 1999 she gave birth to her fourth child, Princess Alexandra of Hanover

Answer: princess caroline of monaco


Question: Category: PARADES 
Clue: The theme of Richard Nixon's second inaugural parade, he thought he'd be in office to celebrate it

Answer: the bicentennial


Question: Category: ARLINGTON'S TOMB OF UNKNOWNS 
Clue: Sentinels at the tomb walk exactly this many steps at a time before they stop & turn

Answer:
21