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: SPORTS 
Clue: Football's imaginary plane which neither team may cross until the ball is snapped

Answer: the line of scrimmage


Question: Category: A DELICATE OPERATION 
Clue: A bypass operation may bypass the carotid or coronary one of these

Answer: artery


Question: Category: 2 
Clue: In "The 12 Days of Christmas", it's the gift there are 2 of

Answer:
turtledoves