一道语法题请教

polaris16251

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

我选了C
date不能用when指代么？ 应该是what is the date? 还是when is the date?
还用我觉得官方选项中的C用了完成时似乎没什么道理

polaris16251

懂了，谢谢啦

soubixl

有个固定搭配叫be to known to do/be to know to have done，后面的和前面的时间一致就是to do，有先后关系就是have done