Scientist 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 milion years ago, pushing back by more than 150,000 years the earliest date when it is known that humans make stone tools

a. when is is known that humans make
b. at which it is known that human had made
c. at which humans are known to have made
d. that humans are known to be making
e. of humans who were known to make

OA after some discussion