（1）     T-7-Q41

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C

这里为什么要用完成时呢

因为make (stone)这个动作在(后人)know之前发生