please help me with the qestion and please tell me why you choose the answer.thang you!___has long been proved ,exercises helps to keep one' figue. A.It B.What C.As D.As it

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please help me with the qestion and please tell me why you choose the answer.thang you!
___has long been proved ,exercises helps to keep one' figue. A.It B.What C.As D.As it

C
as 引导的非限定性定语从句,放在句首.

A

C,as 就像,it省略,exercises代替it