Suppose that cigarette smokers create a negative externality. Further suppose that the government imposes a tax on cigarettes equal to the per-unit externality. What is the relationship between the after-tax equilibrium quantity and the socially optimal (efficient) quantity of cigarettes?
A. They are equal
B. The after-tax equilibrium quantity is greater than the socially optimal quantity
C. The after-tax equilibrium quantity is less than the socially optimal quantity
D. There is not enough information to answer this question.