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  3. As the earth’ s surfaces warm, evaporation(蒸发)is drying out forests and soils, increasing susceptibility to fire. Last summer, m

As the earth’ s surfaces warm, evaporation(蒸发)is drying out forests and soils, increasing susceptibility to fire. Last summer, m

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问题     As the earth’ s surfaces warm, evaporation(蒸发)is drying out forests and soils, increasing susceptibility to fire. Last summer, more than 7.3 million acres of U.S. forests burned during an intense drought. Most alarmingly, as an intergovernmental panel concluded in 2001, earth’ s biological systems are already responding to climate change. The current epidemic(传染病)of bark beetles adds a new dimension to the risk of fires.
    Mountain bark beetles attack lodgepole, ponderosa(黄松), Douglass fir(枞木), sugar and western white pines, destroying them by injecting a fungus(真菌). The galleries of eggs they lay inside the bark pave the way for the trees’ death within a year. Healthy trees secrete pitch to drown the invaders and plug the holes they bore, but drought dries out the pitch. Woodpeckers and nuthatches keep adult numbers in check, but with warmer winters, beetle populations can quadruple in a year, outpacing their pursuers.
    Warming is increasing the reproduction, abundance and geographic range of beetles, destabilizing the age-old, hard won truce between insects and vegetation. Since 1994, mild winters in Wyoming have helped the beetle larvae(幼虫)survive the season. Usually, 80 percent die, but the mortality rate has dropped to less than 10 percent. In Alaska, spruce bark beetles are sneaking in an extra generation a year due to warming, and have destroyed 4 million acres in the Kenai Peninsula in the past five years. "This is another example of global climate change that has deadly implications for my state," declared Alaska’s Republican Sen. Ted Stevens last year.
    Warming is also expanding the beetles’ range into higher altitudes. In the past four or five years, they have begun to attack whitebark pines at an elevation of 8,000 feet or higher. Jesse Logan, who works for the Utah Forest Service, told the Billings(Mont.)Gazette last month that this development coincides with an overall wanning trend that began in the 1980s. "Beetles are cold-blooded, so their metabolism is related to the environment they’re in," according to Logan, who said the beetles seem to be a reliable indicator of global climate change.
Why do we have more wildfires now than before?
选项 A、Because the number of the bark beetles is decreasing.
B、Because trees where bark beetles live dry out easily and catch fire.
C、Because bark beetles prevent the earth from getting warmer.
D、Because bark beetles are cold-blooded and need fire to warm up their bodies.
答案B
解析 推断题。根据文章的第一段，气候变暖导致树木及土壤水分量减少，导致火灾数量的增多。由此可见问题在于水分的减少。而这种害虫导致树木死亡，死亡的树木含水量很低，就更容易着火，故选B。
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