[求助]GWD21 34有关date后面定语从句引导词

Deboisseu

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

这个题目一点不知道从何下手，答案是A。请各位帮忙分析下。

date后面的定语从句应该用哪个词引导啊？at which还是when?还是都可以？

morespring

date 后面跟at which 或 when 都是可以的，OG12上有讲

这一题可以肯定得是A是错的，答案应该是C

sb is(are) known to have done sth是最佳的句型，参见prep第83题

The number of plant and animal species that humans are known to have forced to extinction in the last half millenium is over 800.

singletonboy

支持，C