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Scientists have dated sharp-edged flakes of stone found in the fine-gr

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dipanjan_1988

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24 Oct 2015, 07:02

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mikemcgarry

ziko

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
Thanks Mike for good explanation. I have chosen B, and think it is grammatically correct, but again here as you have explained "it is known" refers to the time when we find out about it but not humans first tool making time. Am i correct or i missed anything?

Dear Ziko,
First of all, yes, there's the logic issue --- the "earliest date" --- does this refer to the earliest date of knowing or the earliest day of making stone tools. Like (A), (B) also doesn't resolve this ambiguity.
Furthermore, (B) using something called the "empty it" ---- the "it" in "it is know that humans had made stone tools" is a pronoun that doesn't refer to any antecedent. It doesn't refer to anything. It is purely a grammatical placeholder, and in that sense it is "empty" --- unlike most bonafide pronouns, this "it" refers to nothing --- it has no valid antecedent. The GMAT generally avoids the "empty it" --- once or twice, I have seen an OA on official material involving an "empty it", but the GMAT uses the "empty it" far more frequently to construct wordy indirect phrases for incorrect answer choices.

Consider these two sentences:
(1) It is known that early humans used stone tools.
(2) Early humans are know to have used stone tools.
Both are grammatically correct, but the GMAT would consider the second one more direct, more powerful, and therefore a much better answer than the first.
Be suspicious of the "empty it" wherever you see it --- even if it's grammatically correct, it is in all likelihood not the correct answer.

Does all this make?

Mike :-)

Hi Mike,
Thanks for the justification regarding the essence of the sentence. Its much clear now. However, I have one question. When referring to a date or a time frame, I learnt that usage of 'when' is preferred over usage of 'which'. That's the sole reason I chose A over C. Can we use both at which and when while referring time? Help me out here please

mikemcgarry

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26 Oct 2015, 10:12

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dipanjan_1988

Dear dipanjan_1988,
My friend, I'm happy to respond. :-)

Of course, it is never a problem to use "when" in reference to a date or time. As concerns the word "which," I would make this distinction. The word "which" by itself sounds extremely awkward.
. . . the day ~~which~~ he came. . .
That should be
. . . the day when he came. . .
So, by itself, "which" is atrociously wrong. BUT, in a preposition construction with "at" or "in" or "by" is perfectly correct and in fact typical of sophisticated writing.
. . . the date at which Prohibition took effect . . .
. . . the year in which Flaubert published Madame Bovary . . .
. . . the time by which Helen will have arrived in Croatia . . .
Those are not only correct: they actually sound more sophisticated than they would have if we used "when."

I would caution you that you cannot master GMAT SC simply by learning some mythical "complete list" of grammar rules. To achieve mastery, you have to read. See this blog:
https://magoosh.com/gmat/2014/how-to-imp ... bal-score/
By reading sophisticated writing, you will develop a sense for the kinds of structures that are typical of sophisticated English writing.

Does all this make sense?
Mike :-)

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15 Mar 2016, 05:45

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iDisappear

Here 'when' is used as an adverb after an expression of time to mean 'at which'.
'Sunday is the only day when I can relax.
The action 'relax' takes place on Sunday.
In the problem the action 'is known' takes place at a particular point of time in the past. It is incorrect. The correct action is 'humans made stone tools'.
Therefore, I think it is correct to say 'when humans made stone tools', or 'when human are known to have made stone tools'.
Both the structures are absent in the options.
But we can replace 'when' with 'at which'.
So, we have the correct option C

bkpolymers1617

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04 Sep 2017, 11:46

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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.

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23 Oct 2020, 02:43

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alimad

This question is based on Construction and Idiomatic Usage.

The adverb ‘when’ indicates the time of an action and so, it cannot modify the noun ‘date’. The simple present tense 'is known' is inappropriate as the date marks the time that humans made tools, not the time at which it was known. The simple past tense 'made' is also incorrect in this sentence as a completed action is to be indicated. The perfect tense would make more sense. So, Option A can be eliminated.

The idiomatic usage of the preposition ‘at’ is appropriate in this option. However, Option B is wordy and the past perfect tense ‘had made’ is inappropriate. The past perfect tense is used to indicate an earlier action when two actions happen in the past. So, Option B can also be eliminated.

Option C has the appropriate idiomatic usage ‘date at which’. The present perfect tense ‘have made’ is appropriate in the sentence as it refers to an action done by humans that still has an effect on the present. So, C is the best of all the options.

Option D has the modifier “that humans are known to be making”, which implies a sense of continuity in the present, that is not intended in the sentence. So, Option D can be eliminated.

This option has the phrase ‘date of humans’, which is illogical. The modifier “who were known to make” implies that there were two groups of humans, one who made stone tools, and one who didn’t. Since this meaning is also illogical, Option E can also be ruled out.

Therefore, C is the most appropriate option.

Jayanthi Kumar.

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22 May 2021, 02:27

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IanStewart please could you weigh in on this one.

Also, is the past perfect tense in (B) appropriate?

IanStewart

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Hoozan

IanStewart please could you weigh in on this one.

Also, is the past perfect tense in (B) appropriate?

About the verb tense here, if you say:

Ten thousand years ago, humans had invented stone tools and had learned to control fire, but had not yet invented the wheel.

then that means: if you go back in time 10,000 years, humanity had already figured out how to make tools from stone, and how to use fire. This sentence doesn't say exactly when humanity figured that stuff out, only that it happened more than 10,000 years ago. It might have been 20,000 years ago, or it might have been (as is actually true) 2 million years ago. If instead you say

Ten thousand years ago, humans invented stone tools and learned to control fire, but had not yet invented the wheel.

then this sentence means: about 10,000 years ago, humans invented stone tools and learned to control fire. The sentence is now describing exactly when humans learned to use stone tools and fire (and the sentence now is no longer true).

In the original question, the sentence is describing the date at which humanity first learned to work with stone tools -- that is, it is trying to convey the same meaning as my second example sentence above, and not the meaning of my first example. So using the past perfect would certainly be wrong here, and B cannot be the right answer.

Otherwise I don't have much to add to what has already been said. The only answers with sensible verb tenses are A and C, and Mike McGarry's top post in this thread perfectly explains the subtle distinction in the meaning of those two sentences. Answer A appears to describe when we knew humans first made stone tools, while answer C describes when humans actually first made stone tools. The sentence means to discuss the latter, so C is right. The other discussion in this thread, about the "empty 'it'" or the use of "when" seems to me to miss the point; there's nothing inherently wrong with how "it" and "when" are used in answer A, and the problem is that A conveys the wrong meaning.

I'd disagree with Mike's later post that questions the source of the problem. The meaning issue between answers A and C is so subtle, and the writing here so precise and detailed, that I'd be surprised if this were not an official problem. It's a much higher-quality question than almost any prep company questions I encounter.

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17 Sep 2021, 09:19

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Can some please explain why "E" is wrong ? The Sentence clearly takes about tool making and from what I have learned, it is not idiomatic to say "date of ..." , the rest of the sentence is correct: SV + tense

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