instruction:
Given a factoid/trivia type question, generate the topic of the question. The topic is the entity the question talks about.
question:
Which is the only Avril Lavigne song to have reached No.1 in the US Billboard Chart?
answer:
avril lavigne


question:
Which biologist was known as Darwin's Bulldog?
answer:
charles darwin


question:
The Oscar winning film 'Aviator' was about which man?
answer:

the aviator