请NN们帮忙看看这道题选什么，TS和GWD给出来的答案一个是A，一个是C

Q41:

Scientists have dated sharp-edged flakes of
stone found in the fine-grained sediments of a dry riverbed in the Afar region
of Ethiopia to between 2.52 and 2.60 million years ago, pushing back by more
than 150,000 years the earliest date when it is known that humans made
stone tools.

A. when it is known that humans made

B. at which it is known that humans had
made

C. at which humans are known to have made

D. that humans are known to be making

E. of humans who were known to make