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  3. Fueled by weather, wind, and dry undergrowth, uncontrolled wildfires can bum acres of land—and consume everything in their way—i

Fueled by weather, wind, and dry undergrowth, uncontrolled wildfires can bum acres of land—and consume everything in their way—i

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问题     Fueled by weather, wind, and dry undergrowth, uncontrolled wildfires can bum acres of land—and consume everything in their way—in mere minutes.
    【C1】______, more than 100,000 wildfires clear 4 million to 5 million acres of land in the U. S. every year. A wildfire moves at speeds of up to 23 kilometers an hour, consuming everything— trees, bushes, homes, even humans—in its【C2】______.
    There are three conditions that need to be【C3】______in order for a wildfire to burn: fuel, oxygen, and a heat source. Fuel is any material【C4】______a fire that will burn quickly and easily, including trees, grasses, bushes, even homes. Air supplies the oxygen a fire【C5】______to burn. Heat sources help spark the wildfire and bring fuel to【C6】______hot enough to start burning. Lightning, burning campfires or cigarettes, hot winds, and even the sun can all provide【C7】______heat to spark a wildfire.
    【C8】______often harmful and destructive to humans, naturally occurring wildfires play a positive role in nature. They【C9】______nutrients to the soil by burning dead or decaying matter. They remove diseased plants and harmful insects from a forest ecosystem ( 生态系统 ). And by burning【C10】______thick trees and bushes, wildfires allow sunlight to reach the forest floor, enabling a new generation of young plants to grow.  
【C8】
选项 A、Although
B、As
C、If
D、Whereas
答案A
解析
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