It has long been accepted that the Americas were colonized by a migration of peoples from Asia, slowly traveling across a land bridge called Beringia (now the Bering Strait between northeastern Asia and Alaska) during the last Ice Age. The first water craft theory about the migration was that around 11,000-12,000 years ago there was an ice-free corridor stretching from eastern Beringia to the areas of North America south of the great northern glaciers. It was the midcontinental corridor between two massive ice sheets-the Laurentide to the west-that enabled the southward migration. But belief in this ice-free corridor began to crumble when paleoecologist Glen MacDonald demonstrated that some of the most important radiocarbon dates used to support the existence of an ice-free corridor were incorrect. He persuasively argued that such an ice-free corridor did not exist until much later, when the continental ice began its final retreat.
According to paragraph 1, the theory that people first migrated to the Americans by way of an ice-free corridor was seriously called into question by
A.paleoecologist Glen MacDonald's argument that the original migration occurred much later than had previously been believed
B.the demonstration that certain previously accepted radiocarbon dates were incorrect
C.evidence that the continental ice began its final retreat much later than had previously been believed
D.research showing that the ice-free corridor was not as long lasting as had been widely assumed
首先，阅读题干了解到问的是：人们第一次移民到美洲通过无冰走廊这个理论被严重的质疑了通过什么内容。那面对这样一个题干，关键词在于called into question。
所以，接下来到文章中寻找关于“质疑”的相关信息点。我们发现But belief in this ice-free corridor began to crumble when paleoecologist Glen MacDonald demonstrated that some of the most important radiocarbon dates used to support the existence of an ice-free corridor were incorrect.这句话比较明显的提到了该理论crumble（崩塌了），其实就是陷入质疑，是因为Glen MacDonald展示出用于得出前文理论的证据是错误的。