During the Ice Ages people migrated across a land bridge from?

During the Ice Age a vast, cold and dry Mammoth steppe stretched from the arctic islands southwards to China, and from Spain eastwards across Eurasia and over the Bering land bridge into Alaska and the Yukon where it was blocked by the Wisconsin glaciation.

Explanation:

Most archaeologists agree that it was across this Bering Land Bridge, also called Beringia, that humans first passed from Asia to populate the Americas. Whether on land, along Bering Sea coasts or across seasonal ice, humans crossed Beringia from Asia to enter North America about 13,000 or more years ago.

Explanation:

d. it adversely affected american productivity