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

date可以用at which修饰？又不是具体哪一天......

another Q is , 需要用现在完成时吗？

and what's the error in A?

tks......

在between 2.52 and 2.60 million years ago,继续pushing back by more than 150,000 years

过去的过去，用完成时态。

GWD 21-34

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

我觉得应该选A, it在这里是形式主语（应该是这么叫吧，还是逻辑主语。。。），是指代后面的that从句。我认为这里不应该用完成时或者进行时，一般过去时就好了。

总算找到原文重现了;呵呵

This so-called Oldowan phase of toolmaking ... years ago, pushing back by more
than 150000 years the known date at which humans were making stone tools

可能是英文习惯用法:THE DATE AT WHICH@@@@@

剩下B,C项.

B项与C项比,明显罗嗦.而且过去完成时说明发生在THE EARLIEST DATE之前就完成了,不合题意.SO......

the date when ,the date at which 在日常用的频率相当，但是在gmat中，较好的用法是at which,其他的还有the age in which, the way in which, phenomenon in which等等。