Text ... Fossil fuels burned to【C1】______cars and trucks,【C2】______homes and businesses, and power factories are【C3】______for

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问题                               Text
   ... Fossil fuels burned to【C1】______cars and trucks,【C2】______homes and businesses, and power factories are【C3】______for about 98% of US carbon dioxide emissions, 24% of methane emissions, and 18% of nitrous oxide emissions. Increased agriculture, deforestation,【C4】______, industrial production, and mining also contribute a significant【C5】______of emissions. In 1997, the United States【C6】______about one-fifth of total global greenhouse gases.
   The 20th century’ s 10 warmest years all【C7】______in the last 15 years of the century. Of these, 1998 was the warmest year on【C8】______. The snow【C9】______in the Northern Hemisphere and floating ice in the Arctic Ocean have decreased. Globally, sea level has【C10】______4~8 inches over the past century. Worldwide precipitation over land has increased by about one percent. The【C11】______of extreme rainfall events has increased throughout much of the United States.
   Increasing concentrations of greenhouse gases are likely to【C12】______the rate of climate change. Scientists expect that the average global surface temperature could rise 1~4.5℉(0.6~2.5℃)in the next fifty years, and 2.2~10℉(1.4~5.8℃)in the next century,【C13】______significant regional variation. Evaporation will increase【C14】______the climate warms,【C15】______will increase average global precipitation. Soil moisture is likely to decline in many regions, and【C16】______rainstorms are likely to become more frequent. Sea level is likely to rise two feet along most of the US coasts.【C17】______of climate change for specific areas are much less reliable than【C18】______ones, and it is unclear【C19】____________regional climate will become more【C20】____________.  
【C14】

选项 A、as
B、while
C、when
D、as if

答案A

解析 本题考查连词用法。根据题意,前后两句之间是因果关系,故选A。
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