求助08语法题一道!Scientists have dated sharp-edged flakes of stone

求助08语法题一道!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.A.when it is known that humans madeB.at which it is known that humans had madeC.at which humans are known to have madeD.that humans are known to be makingE.of humans who were known to make求问这道题A怎么错了?