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  3. We all know (or should know) by now that the carbon dioxide we produce when we burn fossil fuels and cut down forests is the pla

We all know (or should know) by now that the carbon dioxide we produce when we burn fossil fuels and cut down forests is the pla

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问题    We all know (or should know) by now that the carbon dioxide we produce when we burn fossil fuels and cut down forests is the planet’s single largest contributor to global warming. It persists in the atmosphere for centuries. Reducing these emissions by as much as half by 2050 is essential to avoid disastrous consequences by the end of this century, and we must begin immediately.
   But this is a herculean undertaking, both technically and politically. There is, however, a short-term strategy. We can slow this warming quickly by cutting emissions of four other climate pollutants: black carbon, a component of soot; methane, the main component of natural gas; lower-level ozone, a main ingredient of urban smog; and hydrofluorocarbons, or HFCs, which are used as coolants. They account for as much as 40 percent of current warming.
   We can reduce black carbon emissions significantly in the next few decades by using particulate filters on cars and trucks and switching to low-sulfur diesel. By employing those strategies, California, for instance, has cut the warming effect from diesel emissions by nearly half since the late 1980s. In addition, we can further reduce emissions of black carbon and carbon monoxide (which produces lower-level ozone) in the developing world simply by turning to efficient biomass cook stoves instead of using traditional mud stoves, by replacing kerosene lamps in villages with solar lamps, and by deploying modern brick kilns.
   Methane emissions can be cut by nearly a third by reducing leaks from gas pipes, coal mines and hydraulic fracturing, by capturing methane from waste dumps, water treatment plants and manure, and by cutting emissions from rice paddies.
   These reductions in methane, carbon monoxide and volatile organic compounds would also significantly reduce lower-level ozone, which is another important climate-warming pollutant that is formed by the interaction of sunlight with other short-lived pollutants.
   And HFCs, which are widely used in refrigerators, can be replaced with readily available climate-friendly refrigerants. Nearly 100 ozone-depleting chemicals have been phased out under the Montreal Protocol, an international treaty that took effect in 1989 , and more than 100 countries support a shift to the safer HFC alternatives. Phasing down HFCs would provide climate protection many times greater than the current Kyoto climate treaty—the equivalent of about 100 billion tons of carbon dioxide by 2050.
   Unlike carbon dioxide, these pollutants are short-lived in the atmosphere. If we stop emitting them, they will disappear in a matter of weeks to a few decades. These reductions would also prevent an estimated two to four million deaths from air pollution and avoid billions of dollars of crop loss annually, according to a study commissioned by the United Nations Environment Program and the World Meteorological Organization.
The reason why the four other climate pollutants can disappear in a matter of weeks is that______.
选项 A、they are short-lived in the atmosphere
B、more than 100 countries support a shift to the safer HFC alternatives
C、nearly 100 ozone-depleting chemicals have been phased out under the Montreal Protocol
D、they account for as much as 40 percent of current warming
答案A
解析 此题为细节分析题。根据线索词disappear in a matter of weeks定位至末段第二句。而其前一句Unlike carbon dioxide，these pollutants are short—lived in the atmosphere，即与二氧化碳不同的是，这些污染物存在于大气中的时间很短，正是对本题的解答。因此，A选项为正确答案。
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