【单选题】Ancestral humans continued their global spread to the New World, reaching North America approximately _______ years ago and the southern regions of South America 15,000 years later.
【单选题】请找出下面段落中的主题句的位置。 Geographers have found that there are some important differences between the northern and southern polar regions of the earth. The Arctic region, surrounded by masses of land, is in t...