【求助】一道语法题-关于it指代和做不做形式主语

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       (C)

E. of humans who were known to make