How were early humans able to cross from asia to the americas?
a.
they traveled down the east coast of north america.
b.
they crossed a strip of land that existed between the two continents.
c.
they built a land bridge that connected asia and north america.
d.
they walked over glaciers during the last ice age.

D.  They walked over glaciers during the last Ice Age.

Explanation:

The settlement of America is the process of dispersion of the human species in the Americas. Scientific studies claim that human beings are not native to the Americas, and it is clear that it was populated by humans from other places. Paleoanthropological and genetic evidence supports the hypothesis that the first settlers of America came from Siberia, the northeast corner of Asia. From the theory of late settlement, Paleoamericans entered the continent during the last glaciation, when it was possible to move to the New World through the isthmus that is currently occupied by the Bering Strait. This strait was covered by ice during the last glaciation, which would have allowed the first settlers to walk over the glaciers and to cross from Asia to North America. This happened between 15,000 and 14,000 years a. C.