Sentence Correction

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


Source: 1000 SC...A bit unreliable..but still..

[spoiler]OA given A

B, D and E straight out...
C looks awkward when compared to A...Everything seems fine to me but still isn't 'it' ambiguous here?[/spoiler]

'it' refers to language here. In (A) 'it' is closer to language and makes sense.