Given a factoid/trivia type question, generate the topic of the question. The topic is the entity the question talks about.
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Question: The Byzantine Empire is the term used since the 19th century to describe the Roman Empire of the Middle Ages, centred on its capital, where?

Answer: byzantine empire

Question: The rivers Axe, Brue, Cary, Parrett, eta, are in which English county?

Answer: castle cary

Question: Who took over from Jeff Stelling as the host of 'Countdown'?

Answer:
countdown