Sentence Correction

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

C looks like the right option

IMO: B
verb is consistent with the rest of the sentence

A/B - 'it' is ambiguous.
D - known to be making is wrong as the sentence talks about that period
E - totally awkward.

IMO C.

GmatKiss wrote: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 million years ago, pushing back by more than 150,000 years the earliest date when it is known that humans make stone tools

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

OA after some discussion

IMO: C

A, B ambiguous IT,

D has tense error,

E is incorrect because of "of humans"

Simply C looks fine.

OA is C