再问大全606，句子谓语的问题

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

答案是A，我只是不明白为何是“has revealed”  而不是have。。。has是谁的谓语？

CDE的话似乎主句就没有谓语的，A是nature of language 和the processes『（that produce and make）it understandable』并列的吧，B我觉得很乱