What does the word precursor in the context in the second paragraph mean？

What does the word precursor in the context in the second paragraph mean？ 查看材料

A.A man that may come before the happening of an earthquake.

B.The leading factor that may cause a series of events to occur.

C.Any meaningful sigh that indicates some would-be happenings.

D.An event that may have symbolic or significant nature.

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