prep 一道语法题

gmatziyu

256. (28362-!-item-!-188;#058&003003) (T-4-Q1 TS-7-41)
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：C
请问为什么不选A呢？谢谢

Bourne_NY

这里实际上是一个过去完成时had made，表当时人类就已经制造石器了。但因为be known to结构，to后面只能跟动词原形，所以就成了to have made。

gmatziyu

it is known 也经常用阿....have made和made在这题中有什么区别么？
谢谢...

Bourne_NY

A里的it is known that humans made冗繁。

humans are known to的表述方式最简洁。