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.
- when it is known that humans made
- at which it is known that humans had made
- at which humans are known to have made
- that humans are known to be making
- 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? |