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
E. of humans who were known to make
Give Explaination plz.
The usage of "it" in A and B is ambiguous ...although we know that "it" points to an implicit subject, "it" can be understood to modify "the earliest date" --> cross A and B
C looks fine: we need to use present perfect " have made" to indicate something happened in past.
D: the action of "make" no longer takes place --> "to be making" is incorrect!
E: "humans" is a broad and general noun --> the use of "who ..." to modify "humans" is unsuitable
Go for C.