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.
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Question: Category: "HEAD" LINES 
Clue: This phrase may precede "above the rest" or "shampoo"

Answer: head and shoulders


Question: Category: THE TITANIC 
Clue: Gates prevented 700 of these passengers from getting up to the main deck, though they didn't stop Leo in the movie

Answer: third class


Question: Category: "ACTION"! 
Clue: 2-word "atomic" term for a series of events in which each event is the result of the one preceding & the cause of the next

Answer:
a chain reaction