Given a factoid/trivia type question, generate the topic of the question. The topic is the entity the question talks about.

Q: Which group of the 1970's featured Tony Iommi on guitar, Geezer Butler on bass and Bill Ward on drums?

A: geezer butler
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Q: Which film was based on the story The Sentinel by Arthur C Clarke?

A: arthur c. clarke
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Q: Who composed the Air for the G string (init and name)?

A:
air on the g string
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