\"Remember... Only YOU can prevent wildfires.\" These are the famous words that appeared on posters with Smokey the Bear, starting in the 1940s. In a way, Smokey was right--humans do start the majority of forest fires. Some are result of an unattended campfire or carelessly discarded cigarette. Others are started accidentally by a downed power line or deliberately by arsonists (n. 纵火犯). Once a wildfire starts, it spreads quickly and is difficult to contain. A wildfire can move at a speed of approximately 14 miles (22.53 km) per hour and burns everything in its path. Every year, more than four million acres (n. 英亩) of land are burned in forest fires in the United States.

Not all fires are started by humans. Lightning storms are a frequent cause. In addition, conditions, particularly in the American West, often create the perfect atmosphere for fires that burn out of control. Firefighters refer to the fire triangle, which are the three ingredients necessary for a wildfire: fuel, oxygen, and a heat source. Although wildfires occur all over the country, they are most common int he West. Frequent droughts (n. 干旱) and dry weather create brittle (adj. 脆), dry trees, leaves, branches, and vegetation (n. 植被) that go up quickly in flames. When forest fires get very large, the intense heat can even change local weather conditions--basically creating its own weather.

Even the wind can aid in the spread of a fire. Santa Ana winds occur in Southern California in fall and early winter. This hot, dry wind can push a forest fire ahead at terrifying speeds. When populated areas are nearby, these quickly moving fires can be a great threat to property, as well as to human life.

Firefighters have several ways of slowing and stopping the spread of a wildfire, although some fires take weeks to get under control. Spraying the fire and the surrounding vegetation with water and the flame retardants is one step. Another vital element is creating a fireline of firebreak, which deprives (v. 剥夺) a fire of the fuel it needs to continue burning. A firebreak is created by clearing a strip of land of brush and vegetation, often with a tool called a pulaski (a combination of an ax and a hoe). When the fire reaches the firebreak, the hope is that it will burn itself out for lack of fuel.

Firefighters also rely on the help of planes and helicopters. They can drop water or chemicals on the fire from above in an attempt to put out the blaze. The benefit of using airborne (adj. 空中) assistance is that it can cover relatively large distance in short periods of time. In addition, firefighters sometimes set controlled burns. They intentionally light fires to clear vegetation and again deprive a wildfire of its fuel.

Although wildfires are enormously destructive--to both natural and human-made habitats-they do serve some purposes in nature. A wildfire returns nutrients to the soil when decaying matter burns. A wildfire can remove layers of think undergrowth that tend to block sunlight from the forest floor. This allows new growth to receive the light it needs. In addition, fires rid the forest of plants that are diseased, as well as bugs that are harmful and destructive. The cost may not be worth the benefit, but like nearly everything in nature, wildfires have a job to do, too.

问题:

1. What three elements compose the fire triangle?

2. What effect do weather conditions have on forest fires?

3. What is the purpose of creating a firebreak?

a. to allow nutrients to return to the soil

b. to deprive a fire of fuel

c. give humans a chance to flee safely

4. What was the author\'s purpose in writing this selection?

5. What are three ways in which humans can cause a forest fire to start?

6. What are benefits of a wildfire to a forest?