TASK DEFINITION: Given a factoid/trivia type question, generate the topic of the question. The topic is the entity the question talks about.
PROBLEM: Which Chuck Berry song is performed by Marty McFly at the end of the film Back to the Future?

SOLUTION: back to the future

PROBLEM: Which popular Doctor Who villains, it was revealed in 2011, are to be given 'a rest' by scriptwriters?

SOLUTION: doctor who

PROBLEM: Which European city hosted the 2004 Summer Olympic Games?

SOLUTION:
2004 summer olympics