单选题 Fueled by weather, wind, and dry undergrowth, uncontrolled wildfires can burn acres of land and consume everything in their way in mere minutes.
11 , 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 12 .
There are three conditions that need to be 13 in order for a wildfire to burn: fuel, oxygen, and a heat source. Fuel is any material 14 a fire that will burn quickly and easily, including trees, grasses, bushes, even homes. Air supplies the oxygen a fire 15 to burn. Heat sources help spark the wildfire and bring fuel to 16 hot enough to start burning. Lightning, burning campfires or cigarettes, hot winds, and even the sun can all provide 17 heat to spark a wildfire.
18 often harmful and destructive to humans, naturally occurring wildfires play a positive role in nature. They 19 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 20 thick trees and bushes, wildfires allow sunlight to reach the forest floor, enabling a new generation of young plants to grow.