________ is all known to all, water is ________ hydrogen and oxygen．A.As; makingB.It; made up ofC.As; posed ofD.Which; consisted of
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C解析As is known to all是一个定语从句，并不是It is known to all that…结构。which引导的定语从句不能放在句首。be posed of, be made up of和consist of均意为“由……组成”。