What Was the Reason for the Early,prolonged Absence of Humans

What was the reason for the early,prolonged absence of humans in the Western Hemisphere?

A) The warm climate of Africa attracted most of the earth's population.
B) Large herds of mammoths made migration to the Americas too dangerous.
C) North and South America had become detached from the continent of Pangaea.
D) Plentiful food made it unnecessary for northern European tribes to seek a different home.