求解一道 GWD ，答案到时是C还是D

zdf6441634

Scientists have dated sharp-edged flakes of stone found inthe fine-grained sediments of a dry riverbed in the Afar region of Ethiopia tobetween 2.52 and 2.60 million years ago, pushing back by more than 150,000years the earliest date when it is known that humans made stone tools.

1. 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

sdcar2010

at which humans are known to have made

is the answer.

zdf6441634

Thank you.