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: What breed of dog is Marmaduke in the newspaper cartoon strip of that name?

Answer: dog


Question: Where on Prince Edward Island, Canada, is Anne of Green Gables located?

Answer: canada


Question: Double vision, the perception of two images from a single object, is properly called what?

Answer:
double vision