Table 10-4 Quantity (Units) Private Value (Dollars) Private Cost (Dollars) External Cost (Dollars) 1 46 21 6 2 44 24 6 3 42 27 6 4 40 30 6 5 38 33 6 6 36 36 6 7 34 39 6 Refer to Table 10-4. Which of the following policies would move the market from the market equilibrium to the socially optimal equilibrium? a. A tax of $4 per unit of output b. A tax of $6 per unit of output c. A subsidy of $4 per unit of output d. A subsidy of $6 per unit of output