单项选择题

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 argu-ment 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 esti-mate the full economic, or "life cycle", cost for a ton of salt at $500, or 20 times the $25 pur-chase 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