Fueled by weather, wind, and dry undergrowth, uncontrolled wildfires can burn 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.【C1】