While there is great debate about just when the FIRST Americans migrat

While there is great debate about just when the FIRST Americans migrated from Asia to North America, modern anthropologists agree that the following made the trip possible:

A) ice holding so much water in glaciers, making it possible to walk from Siberia across what is now the Bering Sea into North America and then down through passageways in the glaciers.
B) large boats facilitating transoceanic contact between the Jomon culture of Northeast Asia and the Valdivia culture of Ecuador.
C) an underground tunnel connecting the northern tip of Asia in Siberia and North America making the earliest American migration possible.
D) large boats facilitating transoceanic contact between Polynesians in modern American Samoa or Tonga and the Pacific coast of modern California.

ANSWER
Answer: A