Given a factoid/trivia type question, generate the topic of the question. The topic is the entity the question talks about.
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Question: Which film was based on the story The Sentinel by Arthur C Clarke?

Answer: arthur c. clarke

Question: Mar Chiquita (Cordoba) is the largest natural salt lake in which South American country?

Answer: mar chiquita lake

Question: Of all the actors who played Doctor Who in the TV series of the same name, who appeared in the most episodes?

Answer:
doctor who