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

In which US state are the Catskill Mountains?
catskill mountains

What book was written by Alexandre Dumas about Edmond Dantes' imprisonment in the Chateau d'If?
alexandre dumas

Agrippina the Younger was the mother of which Roman Emperor?

agrippina the younger