Answer:
Quaternary period
Explanation:
The last Ice Age on Earth occurred during the Quaternary period. This Ice Age only ended around 10,000 years BC, so very recently. During this period, big portions of the Earth were under ice, with approximately half of North America, as well as half of Eurasia being covered in ice. The mountains around the globe too were covered in glaciers. Because there was so much water trapped in the ice sheets and glaciers, the sea levels were much lower, which opened up lot of land-bridges between the islands and continents, which turned out to be crucial for the human migrations around the globe.
Plains<span> occur as lowlands along the bottoms of </span>valleys<span> or on the doorsteps of mountains, as coastal </span>plains<span>, and as plateaus or uplands. In a </span>valley<span>, a </span>plain<span> is enclosed on two sides but in other cases a </span>plain<span> may be delineated by a complete or partial ring of hills, by mountains or cliffs.</span>
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d. continuous urban development connected by transportation routes.
