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

The Church of the Savior on Spilled Blood, built on the site of the assassination of Tsar Alexander II, is in which Russian city?
church of the savior on blood

Which river begins in the Rocky Mountains in south-west Colorado, crosses New Mexico and forms the border between Mexico and the USA before reaching the Gulf of Mexico?
new mexico

Who played Philip Marlowe in the 1946 film 'The Big Sleep'?
the big sleep