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

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.

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.

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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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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