大全987的606题求教

Research during the past several decades on the nature of language and the processes that produce and make it understandable has revealed great complexity instead of underlying simplicity.

(A) that produce and make it understandable has revealed great complexity instead of underlying simplicity

(B) of producing and understanding it have revealed not underlying simplicity but great complexity

(C) by which it is produced and understood has revealed not underlying simplicity but great complexity

(D) by which it is produced and understood have revealed great complexity rather than underlying simplicity（A）

(E) by which one produces and understands it have revealed great complexity instead of underlying simplicity

大全选A，我选C。解释说C的it不知所知，那A的it又是指什么呢？两个it不是一样的吗？而且A的省略it的用法挺怪的。

谢谢