Given an trivia question precisely answer the question with a word/phrase/name. External resources such as Wikipedia could be used to obtain the facts.
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Question: What word is used in mathematics to mean if and only if?

Answer: if and only if


Question: Which London Underground line connects Euston and Waterloo?

Answer: northern line


Question: Who directed Leonardo DiCaprio in the 2011 film 'J Edgar'?

Answer:
clint eastwood