<em>North America is a geographically diverse continent with a variety of physical features and social structures. The United States and Canada have mountain ranges along their eastern and western portions, with lowlands in the middle. In general, temperatures get cooler as you move from south to north, and the climate gets more arid as you move from east to west across the continent. The mountains of Central America are mainly connected to the greater chain of mountains called the Rocky Mountains, which stretch across North America. Specifically, however, they belong to the Sierra Madre mountains, which is Spanish for the Mother Mountain Range.</em>
What finally enabled sturdy skyscrapers was sturdy steel, which is a mixture of metals and other elements, such as carbon, which is distinguished by low cost on one hand and strength and flexibility on the other.
<span> The correct answer is variable ratio schedule.</span>
