When the Marginal Social Cost of Producing Good a Is

When the marginal social cost of producing Good A is greater than the marginal private cost of producing Good A, then

A) a competitive, unregulated market produces the efficient quantity of Good A.
B) a competitive, unregulated market produces more than the efficient quantity of Good A.
C) a competitive, unregulated market produces less than the efficient quantity of Good A.
D) the government should levy a tax on the production of Good A that is equal to the horizontal distance between the two marginal cost curves.