On average, more than 100,000 wildfires, also called wildland fires or forest fires, destroy two million hectares of land in the U.S every year. A wildfire moves at speeds of about 20 kilometers an hour and can last for weeks, consuming everything - trees, brush, homes, and humans - in its path.

There are three conditions for a wildfire. Oxygen is necessary for a fire to bum. Heat sources help start the wildfire. Although most wildfires are started by people, natural factors like lightning, hot winds, and even the sun can all provide enough heat to start a wildfire.

Firefighters fight wildfires by removing the conditions for a wildfire to bum. Traditional methods include using water and fire retardants to put out the fires. Firefighters also fight wildfires with “controlled burning”. These controlled forest fires remove small trees, bushes, and ground litter from a forest. This will deprive a wildfire of fuel and prevent future forest fires.

Although often harmful and destructive to humans, wildfires play an important role in nature. Even healthy forests contain dead trees and decaying plant matter. When a fire turns them to ashes, nutrients return to the soil. They also remove sick plants and harmful insects from a forest. And by burning through thick leaves and branches, wildfires allow sunlight to reach the forest floor, making it possible for a new generation of trees to grow.

21. What does the phrase “deprive a wildfire of fuel” in the third paragraph mean?

A. create fuel for a wildfire                        B. provide a wildfire with fuel

C. save fuel for a wildfire                        D. take away fuel from a wildfire

22. What does the word “they” in the last paragraph refer to?

A. wildfires                B. forests                C. ashes                D. nutrients

23. According to the last paragraph, why do wildfires play an important role in nature?

A. They bum all leaves and branches.                B. They bum sick plants and harmful insects.

C. They bum a new generation of trees.        D. They bum nutrients in the soil to ashes.

24. Which of the following is NOT mentioned in the passage?

A. Benefits of wildfires                         B. Causes of wildfires

C. Worldwide wildfires                         D. Controlled wildfires

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