Sentence Correction

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


OA C

Source: GMAT Prep

There are many points that the author tried to raise in this sentence. The first is the fact that humans made tools 2.y million years ago, and people knowing that tools were made at that time.

Option A - Incorrect
This is illogical as it explains that people knew that tools were made as far back as 2.6 million years ago, whereas, that was when tool-making started.

Option B - Incorrect
There is a wrong usage of tense here. The sentence is non-idiomatic; it suggests that the knowing doesn't happen anymore. It is also referring to the scientist.

Option C - Correct
'to have made' is parallel to 'scientists have dated'. It also uses the correct form of the idiom 'dated at' instead of 'date of'.

Option D - Incorrect
The use of 'to be making' makes the sentence wrong, and not parallel to the given sentence. Also, the use of the wrong tense and idiom.

Option E - Incorrect
The wrong use of the phrase 'date of' i.e date is mostly used for events not directly with people or living things. For example, 'the date at which the payment was due...'