GMAT Sentence Correction (SC) | Re: Research during the past several decades on the nature of

Research during the past several decades on the nature of language and the processesthat 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