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# sharp-edged flakes

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Senior Manager
Joined: 30 Jun 2011
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08 Jul 2012, 10:19
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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

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

what is wrong with A

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Manager
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08 Jul 2012, 16:48
vikram4689 wrote:
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

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

what is wrong with A

You asked what is wrong with A. Answer choice A results in "...pushing back by more than 150,000 years the earliest date when it is known..."

The earliest date that something is known can't be pushed back.
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Senior Manager
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08 Jul 2012, 18:06
can you elaborate because C also pushes back the earliest date. Ron (mgmat expert) said that clause "it is known" is faulty but i did not get it, may be you can explain me

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08 Jul 2012, 23:09
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.
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28 Aug 2012, 21:23
rjacobsMGMAT wrote:
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,

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29 Aug 2012, 08:25
"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.
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Re: sharp-edged flakes   [#permalink] 29 Aug 2012, 08:25
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