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

I think I will eliminate "A" & "B" because of "known that". The idiom " known to " sounds better. So I probably pick "C" or "E".

Please help me to correct if I was wrong !

Is "know to" a correct idiom?
Still confusing.....
Can anyone help, please~~~~~~~~~~~~~~~~~~~~~~

I will give it a try

A. it does not have a antecedent

B. it does not have a antecedent

C. Correct

D. "known to be making" seems as men will make stone tools in future

E. meaning absurd..seems some date of humans

this is one of those problems from which you can learn about the ways in which these sorts of constructions are used. indeed, it is precisely from these sorts of problems that you must learn about the niceties of these constructions, as the gmat's usage can, and does, sometimes vary from that of other sources.

in this sort of construction, i'm pretty sure that either "when" or "at which" would be acceptable. (clearly, the latter is acceptable -- it appears in the correct answer, after all -- but i wouldn't object to the former.)
if you want to get really subtle, i think (not 100% sure) that "the date when" is used for actual, precise calendar dates, whereas "the date at which" is used for the usually more vague dates of historical events, such as the one in this problem. but i'm sure the test is not going to depend on this sort of nuanced difference.

i think the real problem with the construction in choice (a) is that the clause following "when" is "it is known". in other words, that choice suggests that the fact is (was?) known at that date, an interpretation that clearly doesn't make sense in context.
in the correct answer, the "at which" is followed by a clause whose subject is "humans", and which describes the actual action that took place at that date. therefore, i think the idea is that this clause more accurately describes the chronology of the events: i.e., the toolmaking happened at that date, and the fact is known now.

as for your other question, this isn't really a perfect tense, because it's actually not a tense at all -- it's an infinitive.
probably the easiest way to go here is to remember this as an idiomatic usage of the construction "known to". if the action is in the present, then you use "known to VERB"; if the action is in the past, then you use "known to have VERBed". as far as i know, these are the only two possible forms.

In the correct answer choice:
"at which humans are known to have made",

How do we justify the usage of "are"? Since the action is in past tense, shouldn't we use "were" instead of "are" ?

Please clarify..

this question has already answered, in the post directly above yours.

see:

Pls identify this, is A or C correct???

Since there is controversy between these two choices.

jason, the correct answer is cited in the original post. in fact, it's cited in the portion that you quoted.
go back and look for “the ans is…”

I think this question is hard.

one point I want to comment here.

SC tests our ability to realize the correct sentence among many wrong choices which are created by distorting the correct sentence. That is why it is possible that the incorrect choice can be correct if it stand alone. This is basic thinking of SC

The point here is that when gmat distort the correct sentence by distorting the modification created by a adverbial, we have a hard question.
in this case, the adverbial " the day at which" makes a modification which is distorted in all choices but C.

Yes, I would agree that looking for those distortions will help you get to where you need to go. It's especially helpful if you can see groups of them together in splits.

_________________
Jamie Nelson
ManhattanGMAT Instructor

Hi Instructors ,

What is GMAT trying to test here ?Are they testing Meaning?

Is the usage of the following :
1) at which ( for vague dates )
2) Know to have ( As Ron Pointed out )

the deciding factor in this sentence?

I am facing difficulty with this sentence ?

it appears that you are asking about things that have already been addressed earlier in this thread. i.e.,the points you've listed as #1 and #2 here are already discussed above.
if you have a specific question about some other aspect of this problem, feel free to ask. thanks.

hi Ron - Just to confirm - "Know that" is wrong idiom right!

Cheers

it depends on the context..

_________________
Tim Sanders
Manhattan GMAT Instructor