求教最新GWD-6-41

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.

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

答案为什么是A而不是C

尊的吗？

厄。其实我比较喜欢c a里面有两个动作，when不知道修饰哪一个

满害人的这个话说得。。。