GWD 21-34
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
我觉得应该选A, it在这里是形式主语（应该是这么叫吧，还是逻辑主语。。。），是指代后面的that从句。我认为这里不应该用完成时或者进行时，一般过去时就好了。
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