How did the meltdown of the ice age promote migration within the continent?

it melted the ice in the valleys, thus promoting movement of early immigrants southward and eastward along the continent.
it promoted the growth of plants, and thus early settlers began moving inward to warmer places in search for better land for cultivation.
it promoted the development of fishing for food, thus settlers followed the coastline toward the south and east of the continent.
it melted the frozen land bridge between present-day siberia and north america thus promoting movement inward in the continent.
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