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  3. In an average winter, highway departments spread some ten million tons of salt to keep roads safe. The corrosive effects are wel

In an average winter, highway departments spread some ten million tons of salt to keep roads safe. The corrosive effects are wel

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问题     In an average winter, highway departments spread some ten million tons of salt to keep roads safe. The corrosive effects are well known, but for years they’ve been dismissed with the argument that at around $ 25 a ton, salt is far cheaper than any alternative.
    Lately, economists have added up the actual costs. The Environmental Protection Agency estimates that salt-induced road damage and vehicle corrosion cost $ 3 billion a year. That does not include damage to underground cables, which adds hundreds of millions to utility bills. Nor does it reflect the cost or salty water leaking into roadbeds, which kills evergreens and poisons streams.
    The various costs probably add another $ 2 billion to the national salt bill. Scientists estimate the full economic, or "life cycle", cost for a ton of salt at $ 500, or 20 times the $ 25 purchase price. That begins to make the switch to calcium magnesium acetate (CMA), perhaps the most promising alternative, look practical. CMA costs $ 600 a ton and has minimal side effect. Some researchers believe the cost can be halved by making CMA from waste cheese whey (乳浆).
    While CMA and other alternatives are being researched, many cities and states are cutting back on their use of salt.
With what topic is the passage mainly concerned?
选项 A、Salt substitute for roads.
B、The research of CMA.
C、Environmental protection.
D、Road and vehicle damage.
答案A
解析 本题是一道问及文章主题类问题。通读全文，我们不难总结出本文的中心主题是关于为保证道路安全减少食盐的使用与寻找食盐替代品的问题。因此，本题的正确答案应是A。
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