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

Example input: Who was the first American President?
Example output: president of the united states
Example explanation: The question talks about the president of the USA.
Q: Which British sitcom about the Home Guard in World War II was written by Jimmy Perry and David Croft and broadcast between 1968 and 1977, running for 80 episodes?
A:
david croft