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 made
B. at which it is known that humans had made
C. at which humans are known to have made
D. that humans are known to be making (C)
E. of humans who were known to make
date可以用at which修饰？又不是具体哪一天......
another Q is , 需要用现在完成时吗？
and what's the error in A?
tks...... | 
