Sentence Correction (SC) | Scientists have dated sharp-edged flakes of stone found in

Here is how I eliminate options.

(A) whenit is known that humans made(there is no reference for it)
(B) at whichit is known that humans had made(same as error in option A)
(C) at which humans are known to have made(correct)
(D) that humans are known to be making(comes down to meaning here. Pushing back the date should refer to the time period humans made the tools. This says something else)
(E) of humans who were known to make (comes down to meaning here also. pushing back date of humans
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