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____,many centuries had passed before man learned to use fir

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____,many centuries had passed before man learned to use fire.
A.Since it is known to us all B.As long as it is known
C.As is known to us all D.What is known to us all
Which one?Why?
选C,但是楼上说的不全对,应该是as 和 which 都可以引导非限定性引导定语从句(有逗号的定语从句),可以指代前面的一句话.但是as有“正如,正像”的意思,可以置于句首句中,which只能是居中