h how large a subsidy is required to reach the socially optimal quantity. First, use the "Segment" tool. Place one end of the segment on the marginal private benefit at the socially optimal quantity and the other end on the marginal social cost at the socially optimal quantity. Then, use the "Label" tool to label it "Amount of Subsidy." A Positive Externality Segment Label Undo Redo x Reset Dollars Marginal Private Co Marginal S Marginal Private Benefit quantity Check