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 ______.