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

Brasilia is the capital but which city in Brazil has the largest population?
brazil

Double vision, the perception of two images from a single object, is properly called what?
double vision

What new radio edit was created of a 2009 song by Britney Spears because the original caused controversy in many English-speaking countries due to its use of a double entendre in the chorus?
britney spears