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

Ex Input:
What is the main river of Brazil?

Ex Output:
brazil


Ex Input:
Who won a Gold Medal for Britain in the men's 100 metres at the 1992 Summer Olympic Games in Barcelona?

Ex Output:
britain


Ex Input:
In which  Star Wars episode does Anakin Skywalker transform into the black-suited Darth Vader?

Ex Output:

darth vader