TEXT D 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. According to the passage, the number of wood-burning stove sold for home use ______.
A．went up fivefold over a six-year period B．rose to seven million during the seventies C．multiplied thirty-five times between 1973 and the beginning of 1981 D．increased by 6,000,000 over an eight-year period