Figure 10-11 Price 63 Supply (private cost) 42 36 Social value (private value and external benefit) 24 15 Demand (private value) 0 240 420 Quantity Refer to Figure 10-11. The socially optimal quantity of output is O a. 240 units, since the value to the buyer of the 240th unit is equal to the cost incurred by the seller of the 24oth unit. O b. 240 units, since the value to society of the 240th unit is equal to the cost incurred by the seller of the 40th unit. O c. 420 units, since the value to the buyer of the 420th unit is equal to the cost incurred by the seller of the 420th unit. O d. 420 units, since the value to society of the 420th unit is equal to the cost incurred by the seller of the 420th unit