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

C,

.........date at which something is known to .....is correct idiom
f.e: date at which the portrait is known to have been sold...

E is unidiomatic, completely.

we can say date of event {birth, death...test} f,e: we can say date of his {her, Ann`s} birth, but 'date of ' is not used directly with people or living things.

Dated at or Date at is correct idiom , so A,D, E are out

in B, "it" is ambiguous;
C looks better

hence C

C is correct.

Given : 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.

I simplified the sentence to:

Scientists have dated tool2 to between x and y yrs ago, pushing back z yrs the date when it is known that humans made tools.

A. when it is known that humans made - un-referenced pronoun "it" - wrong
B. at which it is known that humans had made - wrong
C. at which humans are known to have made - Correct
D. that humans are known to be making - wrong
E. of humans who were known to make - wrong

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

Subject is date => at which is correct. Either B or C
It is known that is unidiomatic.
Correct option C.

There is a verb tense issue here. "...when it is known that humans made stone tools" switches from present to past. To change tenses within a clause, you need an explicit change of time period or subject. Since (A) has neither, it is wrong.

(C) solves this problem by using only one verb ("are"). "To have made" is what's called a perfect infinitive, so we don't need to match tense since it's not conjugated.

To be honest though, I liked Ron's explanation a lot. What he's saying is that (A) implies that 150,000 years ago, people knew that humans made stone tools, whereas 160,000 years ago, people didn't know that. This is clearly not the intended meaning of the sentence. Unfortunately though, that's just based on the definition of the word "when" - there's no larger rule here.

Hi,

In option C, is it right to use "at which" for date. I found the usage a little awkward.

Thanks,

"At which" is fine in (C). I could also say "We'll decide at that date." Replace "date" with "time" and you'll notice it sounds OK - these two words work the same way idiomatically.

I was not able to decide between A & C.

Can any1 explain the though process behind answering this 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,
Subject: Scientists
Verb: have dated

pushing back by more than 150,000 years the earliest date when it is
known that humans made stone tools.
Subject: the earliest date
Verb: is

that humans made stone tools
Subject: humans
Verb: made

Thank you Mike, that helped. I got that the "knowing" part is down right now.

Just one confusion

Do we have 2 independent clauses here?

Independent clause 1
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,

Independent clause 2
pushing back by more than 150,000 years the earliest date when it is known [that humans made stone tools.]

Good question. No, there is only one independent clause, the first. A clause must have a bonafide subject and a bonafide verb --- this clause has the subject "scientists" and the verb "have dated."

The second, "pushing back by more than 150,000 years the earliest date ..." is a participial phrase. It has NO subject, and instead of a full verb (e.g. "pushes", "is pushing"), it just has a participle. See this blog: https://magoosh.com/gmat/2012/participle ... -the-gmat/.

What is a bit suspect about this sentence ---- ordinarily a participle would modify a noun, the noun it touches (the "Modifier Touch Rule"). Here, the participle modifies the action of the entire preceding phrase: this is a form that the GMAT SC tends to avoid. The question at the top is not attributed to a source. I would be suspicious of whatever source produced this question. There are so many bad sources of GMAT SC questions out there, and I think this may be from one of them.

Does all this make sense?

Mike

Thanks Mike for good explanation. I have chosen B, and think it is gramatically 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

Thanks Mike,
Your answer is very good. But does it mean that i alsways should avoid "emty it"? (of course whenever i am faced to two gramatically and logically correct answers).

Don't mechanically avoid the "empty it" --- nothing in grammar is completely mechanical. Most often, you will notice that the "empty it" phrasing is longer, wordier, less clear, less concise, less powerful. Most often, if two ways to say something are grammatically correct and one involves the "empty it", then then other will be more concise and more powerful. There are rare cases in which the most direct and efficient way to express something involves the "empty it." There are also GMAT SC problems in which four of the answers are incorrect, and the only possible correct answer involves an "empty it" --- remember, the answer to a GMAT SC question is going to be the best answer from among those five, not necessarily the very best way to express that idea. It will be grammatically correct, but not necessarily ideal.
Does all this make sense?
Mike

A. when it is known that humans made - when modifies "known", seems to suggest it was known 150,000 yrs ago
B. at which it is known that humans had made - same issue
C. at which humans are known to have made - correct answer
D. that humans are known to be making - known to be making is not crrect
E. of humans who were known to make - earliest date of humans doesnt make sense

Hello Mike -

I am trying to understand the logical mistake for (A) being wrong.

When it says "...pushing back by more than 150,000 years the earliest date when it is known that humans made stone tools.

Doesn't it pushes back the date of knowing. Something like,

2.6mn Yrs|..............150k yrs..................|2.45mn yrs......................................................................|............150k yrs..................(push back out earliest date of knowledge).....|Now
<--------------------------------------------------------------------------------------------------------------------------------------------------Pushed back the date of knowing, which is wrong, and hence the wrong answer. Because author's intended meaning in this sentence is about updating our knowledge about first use of stone tools which we thought to be 2.45 mn years old but are actually 2.6 mn years old?


Regards
Sandeep

Dear Sandeep,
My friend, I think you are missing the ambiguity. Think about this phrase:
the earliest date when it is known that humans made stone tools
Let's think about the particular date discussed. This could be interpreted two ways:
1) on the date discussed, humans started making stone tools, or at least leaving tangible evidence for doing so; at some much later time, we figured out from the evidence when this date ways.
2) on the date discussed, it was the first time that humans in history every had awareness that stone tools had ever been used. Now, we take for granted that, at some point in the distant past, folks used stone tools, but at some point in civilization, this was a new discovery, and the date discussed is the date of this discovery.
Of course, what the author means to say is meaning #1, but the grammar doesn't not uniquely support that meaning. On the GMAT SC, we are not allowed to give a sentence the benefit of the doubt. A good sentence says exactly what it means and means exactly what it says, and version (A) does not do this.

Does this make sense?
Mike