In an age when waste today means a lack tomorrow, making use of every available resource becomes more and more important. As coal is being used in greater and greater amounts to produce electricity, larger amounts of ash, by-product of coal, are produced. When coal is burned in a boiler, two kinds of ash by-product are produced, a heavy bottom ash and a fine-as-powder fly ash that is filtered and captured by precipitators. About 10 to 15 percent of the coal by-product is bottom ash which is used like sand on icy city streets and highways and also on highways as paving material. It is the fly ash, however, that is receiving the greater amount of attention. Fly ash may be used as an additive to concrete in the construction of dams, bricks, and roads, and can replace up to 20 percent of the cement used in concrete. As a by-product of burned coal, fly ash requires no additional expenditure of energy to be produced, while cement production requires great amounts of energy.In addition to using fly ash as a concrete additive, the technology is available to extract the main mineral components-aluminum, silicon, and magnetic. Today, extracting these components is a very expensive process, but the time may come when it is cheaper to extract these products from ash than to mine new supplies. Fly ash can partly take the place of ()

A. paving material
B. cement
C. concrete
D. the coal by-product