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

[Q]: Which is the only Avril Lavigne song to have reached No.1 in the US Billboard Chart?
[A]: avril lavigne


[Q]: Who plays Isobel Crawley in the UK television series 'Downton Abbey'?
[A]: downton abbey


[Q]: ..A tale of the riots of eighty completes the title of which work by Charles Dickens?
[A]:
charles dickens