jabhatta2 the trouble with your interpretation of both C and D is that you're reading them in a disjointed manner, without connecting them to the argument above.
It is presently believed that glaciers prevented human migration south from the Alaska-Siberia land bridge between 18,000 and 11,000 years ago.
You have to read C and D in that context. These answer choices are offering up new evidence that (slightly) modifies the prior belief.
I wouldn't go as far as to say that these answer choices claim the premise was a lie - rather, they're perhaps adding a layer of accuracy to the premise.

Surely when you first read this sentence:
glaciers prevented human migration south from the Alaska-Siberia land bridge between 18,000 and 11,000 years ago
You couldn't help but notice that 18,000 and 11,000 are suspiciously round numbers - right? And, 11,200 is dangerously close to 11,000 - what if there's a rounding error?? That's what this problem is playing on.
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Hi MartyTargetTestPrep

I thought the OA stated the premise (marked in red color above) was wrong.

Premise : migration CANNOT take place 18,000 years ago to 11,000 years ago
OA : D -- actually migration DID take place 11,400 years ago and before.

This is a strange OA because we are not allowed to question the premise (in red)

I have seen many answer choices marked INCORRECT in other CR's because the answer choice (a) 'challenged' the premise or (b) the answer choice stated something different to what the premise states

How come in this CR - the D and the premise, both cannot be true at the same time ?

Hi IanStewart MartyTargetTestPrep - my question is if the following WOULD HAVE been another weakeness if the GMAT introduced another option

I ask to solidify my understanding



Normally I would NOT have considered this a legitimate weakeness because this weakness doesnt touch on the 'link' between the premise and the conclusion in the argument.

But I think, in this case this weakness is passable because of the question stem

I think the word "new evidence" mentioned in the question stem -- allows for new evidence to be introduced that doesnt touch on the 'link' between the premise and the conclusion

True you think or you think in this case too -- option F would be rejected because the evidence may be recently discovered but this new evidence still has to touch on the 'link' between the premises and the conclusion.

thoughts ?

If the question was asking specifically about North America, then your answer F would be a perfectly good weakener, because it would point out a possibility the argument hadn't even considered. There might be other ways to get into North America besides through the Alaska-Siberia land bridge, and perhaps people first came by those other routes between 11,000 and 18,000 years ago. But as the question is written, your answer is not a weakener; instead it's a lot like answer B. Brazil and Argentina are part of "the Americas" (the Americas includes South America), so your answer F just establishes that people were already in the Americas more than 13,000 years ago. Maybe they first got there more than 18,000 years ago, and maybe they didn't.
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Thank so much IanStewart -- just so i am clear ...let me scratch out Brazil and Argentina and instead say Asia, that way there is no chance of any overlap



you think this would be a weakener ?

i was debating if this would be a weakener ..

One one hand -- this does talk about something that the argument has not even considered
On the other -- it doesnt weaken the 'link' between the premise and the conclusion specifically

IanStewart
^^ reason is -- on other CR questions - i have seen answers rejected because the answer supposedly doesnt touch on the 'link' between the specific premise and the conclusion

You're right, jabhatta2: generally premises are not to be questioned, and this particular problem appears to violate that rule.

However, let's take a closer look at some language hints that make it okay (I'm guessing that this is how GMAC would defend their choices):
Observe the difference between these two premises:
(1) Charcoal from a hearth site in Colorado, 2,000 miles south of Alaska, is known to be 11,200 years old.
(2) Researchers reasoned that, since glaciers prevented human migration south from the Alaska-Siberia land bridge between 18,000 and 11,000 years ago

So the first premise is "known to be", whereas the second premise is something that "the researchers reasoned"

Then, in the OA, the word "new" is very important:
Using new radiocarbon dating techniques

I hope this helps!
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Which of the following pieces of new evidence would cast doubt on the conclusion drawn above?

(A) Using new radiocarbon dating techniques, it was determined that the charcoal from the Colorado site was at least 11,400 years old.
This doesn't make any difference to the inference therefore out

(B) Another campsite was found in New Mexico with remains dated at 16,000 years old.
Absolutely irrelevant we are weak at geography in addition there is nothing we can infer about the same from the sentence therefore out

(C) A computer simulation of glacial activity showed that it would already have been impossible for humans to travel south overland from Alaska 18,500 years ago.
This further adds strength to the subject therefore out

(D) Using new radiocarbon dating techniques, it was proved that an ice-free corridor allowed passage south from the Alaska-Siberia land bridge at least 11,400 years ago.
This immensely cast doubt on the subject that the passage was absolutely impossible to transverse , no more the case therefore our answer

(E) Studies of various other hunting-gathering populations showed convincingly that, once the glaciers allowed passage, humans could have migrated from Alaska to Colorado in about 20 years.
This doesn't have any impact at all therefore out

THerefore IMO D