T-9-Q27. 大全987-606

T-9-Q27.
    大全987-606

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

不知爲何找不到以前的討論啊

A和C裏面，C爲什麽不對呢？又簡潔又平行，A中that produce and make it understandable在我看來毫無章法啊

请解答