天山7－41请教，都搞晕啦

T-4-Q1
                天山-7-41

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.

1. when it is known that humans made
2. at which it is known that humans had made

3. at which humans are known to have made

4. that humans are known to be making

5. of humans who were known to make

答案：C

我选的是A,而答案是C,为什么选C呢，我觉得A也可以啊，而且不需要完成时，请哪位NN能解释一下？

多谢！