求问一道gwd

Chrislvzh

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.

答案为d ....不大理解...谢谢