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.

答案是d, 不明白~

这题讨论过：结论说一定是比18000年前更加早

时间段：（早）18000前--11400前--11200前--11000前（晚）

D 说某项技术能肯定，至少在11400到18000年之间（包括18000年前之前），已经可以走人了，所以未必就一定是18000年之前的人才是带来那些木炭的。

是这样理解哦~~

我是觉得18000年以前包含在11400年以前之中，也就是说，即然已经在

11400年以前了，就肯定在18000年以前的

所以并没有cast doubt呀~

即然已经在11400年以前了，就肯定在18000年以前

错！11400年前确实包括18000年前，但是11400年至18000年之间这段时间也属于11400年前但却不属于18000年前