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  3. As the cost of gas and oil for home heating has gone up, many Americans have switched from these fuels to wood for heating their

As the cost of gas and oil for home heating has gone up, many Americans have switched from these fuels to wood for heating their

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问题      As the cost of gas and oil for home heating has gone up, many Americans have switched from these fuels to wood for heating their homes. In 1973, approximately 200,000 wood-burning stoves, intended for home use, were sold in the United States; by 1979, this figure had reached one million; and by the end of 1981, there were as many as seven million home owned wood-burning units in operation in the U. S.
     In addition to low fuel bills, many people choose these stoves because their initial cost is very low (the prices range from $ 50 kits to $ 5,000 top-of-the-line models), and because new technology has made wood fires more efficient, cleaner, and, therefore, safer than ever before.
     One new technological feature of this type is the catalytic(催化) combustor which adds about $ 100 to $ 200 to the cost of the stove, but which causes much more complete combustion (燃烧) of the wood and therefore bums up more of the pollutants left by incomplete combustion and produces more heat.
     A second cost-saving innovation is a device which agitates the wood, increasing the amount of oxygen that reaches the center of the wood pile, and leading to more efficient combustion. The real advantage of this device is that it allows the owner to make use of cheap sources of wood such as dirty wood chips (an industrial by-product) that have almost no commercial value, cost as little as $ 20 a ton, and burn very inefficiently in furnaces without an agitator.
The passage states that  ______.
选项 A、many Americans have switched from hydrocarbon-based fuels to wood because the price of the latter has risen
B、some wood-burning stoves cost up to one hundred times more than others
C、catalytic combustors increase the amount of pollution caused by wood burning stoves
D、agitators are cheaper addition to wood-burning stoves than catalytic combustors
答案B
解析 A项表示与事实正好相反，C项应是decrease，而不是increase；D项文中并未说明agitators的价格，所以无法比较。而文中第二段括号内的文字(the prices range from $ 50 kits to $5，000 top-of-the-line models)表明炉子价格之差相差100倍，所以B项是正确的。
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