gwd里的，不知道第几道，好心人帮忙进来看看

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?

1. Using new radiocarbon dating techniques, it was determined that the charcoal from the Colorado site was at least 11,400 years old.
2. Another campsite was found in New Mexico with remains dated at 16,000 years old.
3. 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.
4. 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.
5. 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.

我选的是C，正确答案是D，想确认下原因

题目结论是一定是18000年前到us的

C是说18500年前不可能，但18500-18000间的500年没说不可能，所以没有削弱结论

D是说至少11400前可能到us，那就不一定是18000年前到us的

是这样吗？

谢谢各位！！！！！！！！！！！！

c是加强,注意C的时态,it would already have been...

结论是18000年前到的US,C是18500前就已经不可能了,即:18500年前就已经不可能,更不要说18000年前了

正确答案是E? 应该是D吧.

D提供了一种在11400前就到达US的可能,因此削弱了原题结论--人们在18000前就到达US了.

当然D是不充分的,由于逻辑题答案不需要充分性,只要提供一种可能能够质疑原结论就可以了.

E is a better answer here

20 years is the very small period compared with ten thounds of years.So if human have the chance to come to US through A-S land bridge in very short time.The conclusion will be undermined definitely.

Choice E include the human and time.

Choice D give us a subordinate choice because of the existence of E.

不好意思，把答案看错了，应该是D。不过E也蛮有道理的，所以还是不懂啊，

I thought here both E and D give us a solution to slove the problem we met successfully.Open to dissucss.

Maybe the error in the choice E is the effectiveness of the passage,which has been porved by choice D.Except this one ,choice E is a good choice in my opinion.But D always stands .That's why  I have not given choice D a disclaimation.