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  3. In an age when waste today means a lack tomorrow, making use of every available resource becomes more and more important. As coa

In an age when waste today means a lack tomorrow, making use of every available resource becomes more and more important. As coa

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问题     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 are produced: a heavy bottom ash and a fine - as - pow-der 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 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 magnets, 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 products.
What is a by - product of burned coal?
选项 A、Aluminum, silicon, and magnets
B、Paving material
C、Fly ash
D、A concrete additive
答案C
解析
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