麻烦解释一下这道题，谢谢

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 made

c. at which humans are known to have made

D. that humans are known to be making

E. of humans who were known to make

Answer is C.

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 made

c. at which humans are known to have made

D. that humans are known to be making

E. of humans who were known to make

Answer is C.

首先date用at which 或when 定语从句修饰排除D，E

相同条件下at which 修饰会比when修饰来得准确 排除A

最后B 和C比较，be known to 结构较简洁，to have done sth也较好说明在一个更早的时期人们已经开始make stone tools 在主句的时间完成之前形成对比。

个人愚见 望指正。