Generate a topic word for the given sentence. A topic word is usually the name of a person, place, animal, concept or object the sentence is focusing on.
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Question: the 1st millennium bc was the last millennium before the common era.

Answer: 1st millennium bc

Question: henan is a province of the people's republic of china.

Answer: henan

Question: in mathematics, the cartesian coordinate system is a coordinate system used to place points on a plane using two numbers, usually called the "x-coordinate" and the "y-coordinate".

Answer:
cartesian coordinate system