Answer This Question Based on the Following Information for a Public

Answer this question based on the following information for a public good. Pₐ and Pᵦ 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. Qₛrepresents the quantity of the public good supplied by government at each of the collective prices. The collective willingness of this nation to pay for the fourth unit of the public good is

A) $7.
B) $6.
C) $5.
D) $3.