Answer the Next Question on the Basis of the Following

Answer the next question on the basis of the following information is for public good.P_a and P_b represent the prices that citizens (a) and (b) ,the only two people in this nation,are willing to pay for additional units of a quantity (Q_c) of the public good.Q_s represents the quantity of the public good supplied by government at each of the collective prices.If only 1 unit of this public good is produced,then the marginal benefit is:

A) $3 and the marginal cost is $9.
B) $4 and the marginal cost is $7.
C) $6 and the marginal cost is $3.
D) $9 and the marginal cost is $5.