疑惑 gwd6--41

Q41:

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                  答案是a.  可e错在哪里呢？望牛牛指点
   

E是 date of humans +定语从句，date of humans 没有任何意思啊 ，人的日期？

A中 date 是 push 的直接宾语，因为有when这个定语，后置了