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

[Q]: Which Chuck Berry song is performed by Marty McFly at the end of the film Back to the Future?
[A]: back to the future


[Q]: Which rank of the peerage is directly below a duke?
[A]: duke


[Q]: Brigadier Lethridge-Stewart belonged to which military organisation in the Dr Who series?
[A]:
doctor who