又没有定论~gwd-21-34

Q34:

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，c怎么不对呢？

我正好也在研究这套题呢

答案是C

A when做定语从句还是状语从句不清楚。

C the best

D be making错

这里还有一个主要问题：A/B都没有指明when/at which是修饰从句中的it is known（知道“人类用石器”的时间）还是humans (had) made（人类用石器的时间）. 属修饰歧义错误。