If pollution is reduced below the socially efficient level:

a. the marginal cost of pollution abatement exceeds the marginal benefit of pollution abatement
b. the marginal cost of pollution abatement is less than the marginal benefit of pollution abatement.
c. the marginal cost of pollution abatement equals the marginal benefit of pollution abatement.
d. nothing can be said about the relationship between the marginal cost and marginal benefit of pollution abatement.

a