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 consistency, answers should be in all lower cased letters.

Q: Category: LARGE EQUIPMENT 
Clue: For smooth travel, ballast tampers & ballast regulators keep the gravel at the right level among these

A: railroad tracks
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Q: Category: SCIENCE GRAB BAG 
Clue: A material called URhGe can be manipulated into being this, lacking electrical resistance

A: superconductors
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Q: Category: COKE'S NON-SECRET INGREDIENTS 
Clue: Regular Coke contains the phosphoric type; Diet Coke has citric, too

A:
acid
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