Most Municipalities and States in the Northeastern United States Use

Most municipalities and states in the northeastern United States use rock salt to "produce" ice-free roadways in the face of winter snowstorms.But using rock salt has several drawbacks: it speeds up corrosion of bridges and cars; it can choke vegetation; as runoff,it is harmful to creeks and rivers; and it is not very effective in de-icing roads at low temperatures.If the marginal private cost of rock salt is $60 per ton and the marginal external cost of rock salt is $10 per ton,the optimal corrective tax is ________.

A) $10 per ton
B) $50 per ton
C) $60 per ton
D) $70 per ton