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

Ex Input:
What's the name of Doctor Who's home planet ?

Ex Output:
doctor who

Ex Input:
In which war was the Battle of Bunker Hill?

Ex Output:
battle of bunker hill

Ex Input:
Who, as the first chairman of the British Rail Board, infamously directed the huge reduction in railway lines across the UK in the 1960s?

Ex Output:
british railways board