单选题 .  A recent study described coal as a "bridge to the future". As the most plentiful fossil (化石) fuel in the world, coal has the possibility for filling a growing portion of the demand for energy. But problems cause some trouble to this promising old fuel.
    Coal is found around the globe, but three countries—the United States, the Soviet Union and China own nearly two thirds of all known coal reserves. At present rates of using coal these reserves would last the world more than 200 years, according to the estimates carefully made. Furthermore, scientists think the world probably has 15 times this much coal.
    While the United States has the largest share—more than a quarter of the 786 billion tons of known world coal reserves, both the Soviet Union and China produce almost as much coal as the U.S. does.
    In addition to the category of known reserves, the U. S. Geological Survey (地质调查所) has calculated that the United States has an estimated 1.7 trillion (万亿) tons of coal at depths of less than 3,000 feet. Unfortunately, much of this coal is not easy to get with present technology or at present prices.
    To produce and use much more coal than we do today, a number of problems will have to be solved: capital at high interest rates, manpower, and transportation, including insufficient rail facilities. Environmental concerns include the proper development of mine sites, possible changes in global climate caused by increased carbon dioxide from burning coal, and the objects that are sent out from coal stacks (烟囱) that wear out buildings, poison lakes, and damage human lungs.1.  One of the advantages of coal is that ______.