Home Heating Central heating became popular only after the Civil War. Typically, coal—burning furnaces (火炉)fueled the early systems.Natural gas had developed into the leading fuel by 1960.Its acceptance resulted in part from its wide uses.Because it comes primarily from U.S.and Canadian fields，natural gas is also less vulnerable(脆弱的)than oil is to War.Oil remains the most important fuel in a few areas.such as New England. Electric heating dominates most areas with mild winters and cheap electricity, including the South and the Northwest.It was made popular at least in the South by the low cost of adding electric heating to new houses built with air.conditioning.Bottled gas.which is somewhat more expensive than utility gas，is the fuel of choice in rural areas not served by utility pipelines.Wood is the leading heating fuel in just a few rural counties. Home heating，which accounts for less than 7 percent of a11 energy consumed in the U.S.has had a commendable(值得赞扬的)efficiency record：from 1978 to 1997，the amount of fuel consumed for this purpose declined 44 percent despite a 33 percent increase in the number of housing units and an increase in house size.The U.S.Department of Energy。however, forecasts mat energy used in home heating will rise by 14 percent over the next two decades.That rise is small considering an expected 2 1 percent increase in the number of houses and the trend toward larger houses. Natural gas and electricity will probably dominate the home heating market for the next two decades.Solar(太阳的)heating never became popular because of cost and limited winter sunlight in most areasin 2000 only 47.000 homes relied on it. 第6题：Natural gas didn’t become the leading fuel until