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

这道题新东方的教材上答案是E,GWD的答案是A,看了以前的讨论,还有B和C的版本,大家看看到底应该选什么 啊?

A, date when, it is known that 正确

B, date at which 错

C, date at which 错

D, making 错

E, date of human 有歧义

I see ,thanks !!!!