at which和when的困惑

   牛牛們，新年好！請幫小弟看下這題，A和C哪个好呢？

(如果C答案改成过去完成时会不会更好些呢？)

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

本人不是牛人，姑且说一下自己的理解：

when一般指一段时间，而at表示一个时间点。另外，A选项的it is known that表达不简洁，从时态上看也应该用完成时，表示在那个时候已经开始制作石质工具了。