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

Ex Input:
Which Chuck Berry song is performed by Marty McFly at the end of the film Back to the Future?

Ex Output:
back to the future

Ex Input:
Which 20th century British prime minister had the longest surname ?

Ex Output:
prime minister of the united kingdom

Ex Input:
In the 1968 film Chitty Chitty Bang Bang who buys the car from garage owner Mr Coggins and rebuilds it?

Ex Output:
chitty chitty bang bang