[求助]T-4-Q1

geranium

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

E.    of humans who were known to make

答案是C，请问A有什么不对？

答案是C，请问A有什么不对？

zyca

A语法应该没什么不对，不过读起来是C顺一点，这大概就是所谓的语言的awkwardness吧。。。
最令人气愤的是，XDF蓝皮书中居然给出了答案E，当初让我百思不得其解，害人不浅啊。(Section TEN的Q14)