scientists have dated sharp-edged flakes of stone found in

[tim]

Please let us know if there is a question on this one.

[Crisc419]

Hi, all
Please can anybody help me with this sentence correction question:

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

The ans is c but I chose a. Why a is incorrect？Shouldn't "date" be followed with "when"? And is it necessary to use past perfect tense as in choice c? I would like to choose the simple past tense.

Thank you in advance for your kind help~

babayu

in A, "when it is known" must modify "date" and so this is not logic because the date is in the past but "it is known " is in present.
in B, "at which" can modify "it is known" or "human had made". both these modifications are not logic. the date is in the past, it can not be in present or past perfect
in C, "at which" can modify both " human are known" and " to have made" . but "at which" make sense only with " have known". logic. correct.
in D, "the date that" is ungrammatical. we need "at". this case is in spoken english.
in E, "the date of human" make no sense,

I have read through this thread, but i cannot understand:

1/ in choice C, why we don't interpret that "at which " modify "human are known"? on the other hand, in choice A and B, we interpret that "at which " and "when" modify "it is known".

2/ if we use logic and then decide that in choice C, "at which" should not modify "human are known", then why we don't use logic in A and B, logically, "at which " and "when" should not modify "it is known".

Could anyone clarify for me?

many thanks.

Cris

[RonPurewal]

"when xxxxx" is ALWAYS talking about THE TIMEFRAME OF "xxxxx".
this is why choice A doesn't make any sense -- because we aren't talking about the time when something is known.

[XihanX914]

can i say that the reference of "it" inside the choice A is ambiguous? since it can refer to the earliest date or to the fact ...

[ToanD19]

can i say that the reference of "it" inside the choice A is ambiguous? since it can refer to the earliest date or to the fact ...

No, I'm pretty sure that "it" in A is just a place holder so we do not have to find its antecedent.

[RonPurewal]

can i say that the reference of "it" inside the choice A is ambiguous? since it can refer to the earliest date or to the fact ...

https://www.manhattanprep.com/gmat/foru ... tml#p49622