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: MYTHOLOGY 
Clue: This Norse god known for his great strength was a protector of peasants & farmers

Answer: thor


Question: Category: IN THE TEENS 
Clue: One of these "wheelers" is a large tractor-trailer truck rig

Answer:
18-wheeler