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: 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


Question: The most northerly part of mainland Australia is in which state?

Answer: australia


Question: Which is the only cactus fruit that is widely eaten?

Answer:
opuntia