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

答案选A，我选C。理由：A的意思不合逻辑，A的意思是“把我们知道人类使用石器的时间推前了150000年”。原意应该是“把已知人类使用石器的时间推前了150000年”。

不知对不对，请大家指点。