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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.
It can be inferred from the passage that for many years salt has been used to keep roads safe because______.
选项 A、salt can keep roads and vehicles in good condition
B、salt is the best choice
C、salt is thought to be comparatively cheap
D、salt can not be replaced by any other alternatives
答案C
解析 本题是一道暗示引申类问题。问我们可以从文章中推断出多年来人们用食盐保持道路安全的原因是什么。根据文章第一段内容，为保证道路安全，公路部门要撒掉大约1 000万吨食盐。食盐的腐蚀作用是众所周知的，但是，多少年来这些腐蚀作用并未被当成一回事，其理由是：25美元左右一吨食盐，比用任何替代品都便宜。因此，本题的正确答案应是C“因为人们认为盐相对来说比较便宜”。
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