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

Hi experts,
I didn't go through all the threads under this question,just the last three pages.I'm terribly sorry if I didn't notice that some other posts could help with my request.

Just want to confirm my POE
option B,D,and E are out for the s-v disagreement--Research...have.
Between A and C :
Even though I couldn't justify the usage of by which,C is out because "it" is not required.From my understanding,the research is on the nature of language and on the processes that produce and