sec19-07，谁能有更好的解释？

7.     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

我想问的是C为什么错误？看了大全的解释（ it 不能understood, 应该理解为it was understood by the processes ，这样句意改变不合逻辑），觉得很模糊，还是不明白。

个人认为, 主动优于被动.  C 是被动,ETS不倾向被动

一个是主动，被动的问题。 而且原句的意思是转折在simplicity， c 的转折变成了complexity。