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

[Q]: Which English king was the son of Edward, the Black Prince?
[A]: edward, the black prince


[Q]: Who served in the Crimean War (1854 - 56), was responsible for suppressing the Taiping rebellion in China in 1860, was governor of the Sudan 1877 - 79, and died there?
[A]: taiping rebellion


[Q]: What is the largest city in China, which is thought to be the most densely populated area in the world?
[A]:
china