[求助]大全606

1.                             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

A中的it指代什么，是language吗？可是前面还有一个单数名词research，为什么不说这里的it有歧义。

到底代词优先指代什么名词？