One theory, suggested by Harvard University, is that humans began cooking their food, the

One theory, suggested by Harvard University, is that humans began cooking their food, thereby releasing nutrients locked in the raw food and reducing the time needed to chew it.
A. which
B. as
C. that
D. what

A. which(答案→)B. asC. thatD. what解析：B 考察定语从句。One theory is that…to chew是主句， suggested by Harvard University是非限制性定语从句，as在引导非限制性定语从句时候，有正如的意思，且通常表示被动。Which没有这个用法。故B正确。

哦，回答的不错