Critical Reasoning

Charcoal from a hearth site in Colorado, 2,000 miles south of Alaska, is known to be 11,200 years old. Researchers reasoned that, since glaciers prevented human migration south from the Alaska-Siberia land bridge between 18,000 and 11,000 years ago, humans must have come to the Americas more than 18,000 years ago.

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.



B. Another campsite was found in New Mexico with remains dated at 16,000 years old.



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.



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.



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.



OA is D why not C??? Pls explain each choice for me , and give me assumption and conclusion of the argument. Thank you

Conclusion: , humans must have come to the Americas more than 18,000 years ago.
Premise: glaciers prevented human migration south from the Alaska-Siberia land bridge between 18,000 and 11,000 years ago
Premise: Charcoal is known to be 11,200 years old.
Cast doubt means weaken...
A) both 11,200 and 11,400 lie between 18000 and 11,000 years ago. Thus conclusion is not weakened.
B) same as A. 11,200 and 16,000 support 18,000 years ago theory.
C) Then humans traveled between 18,500 and 18,000 years ago. Still not weakened.
D) As, even 11,400 years ago, there was a passage, we can't say humans came 18,000 years ago. Correct
E) Irrelevant

IMO D

Rephrase: -->
What we need to find out in the argument is a reason where we could tell that human could come in the time span less than 18,000 years ago.

Choice D does that exactly.

Other choices are not that explicit at all.

Thanks for good question.

IMO D.

Thank you guys, thanks for the help. I bless this forum, other gmat forums go to hell.

Conclusion:
Humans must have come to the Americas more than 18,000 years ago

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 states that it was possible for humans to travel south only before 18500 yrs ago.
Difference between conclusion and this statement does not weaken the conclusion just increases the years.We need somethign which will state that humans could have come to Americas even after 18000 yrs ago.

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 proves that the ice-free corrider could have allowed humans to come befreo 11400 yrs ago. this directly breaks the conclusion of the passage. Correct.

Conclusion: Humans must have come to the Americas more than 18,000 years ago.
Reasoning: A blockade prevented humans traveling south between 18,000 to 11,000 years ago. If some information showed that the blockade was passable, then the new evidence would cast doubt on the conclusion. Correct answer weakens the conclusion; incorrect answers are neutral to the conclusion or strengthen the conclusion.
A. Neutral answer. If the charcoal was exactly 11,400 years old, then the conclusion is weakened. If the charcoal was more than 18,000 years old, then this would have strengthened the conclusion. It can go both ways.
B. Neutral. The correct answer must prove that people crossed in between 18,000 and 11,000 years ago. This proves that someone was there 16,000 years ago; the humans could have crossed 2,000 years prior, around 18,000, or sometime shortly after 18,000 years ago.
C. Strengthens. If it was impossible to travel south overland from 18,500 years ago, then the conclusion would be strengthened.
D. Correct. If it was possible to cross at least 11,400 years ago, it provides doubt that the glacier prevented all human migration between 18,000 and 11,000 years ago. Hence, D is the answer.
E. Out of scope/Neutral. This does nothing the conclusion.