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.
- 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
答案是A，但是it is known that 怎么解释呢，难道是没有实际意义的插入语吗 | 
