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

• paleoecologist Glen MacDonald's argument that the original migration occurred much later than had previously been believed
• the demonstration that certain previously accepted radiocarbon dates were incorrect
• evidence that the continental ice began its final retreat much later than had previously been believed
• research showing that the ice-free corridor was not as long lasting as had been widely assumed

This is a multiple choice.