If the production of a good causes an external cost, then the efficient quantity is

• A.equal to the quantity at which the marginal benefit equals marginal cost.
• B.less than the quantity at which the marginal benefit equals the marginal cost.
• C.more than the quantity at which the marginal benefit equals the marginal cost.
• D.the quantity at which the marginal private benefit is greater than the marginal social benefit.
• E.None of the above answers is correct.