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

[Q]: Of all the actors who played Doctor Who in the TV series of the same name, who appeared in the most episodes?
[A]: doctor who


[Q]: Carpo and Elara are moons of which planet?
[A]: elara


[Q]: Which city will host the 2016 Olympic Games?
[A]:
2016 summer olympics