-The Figure Above Shows the Market for Milk in Cowland

-The figure above shows the market for milk in Cowland. A subsidy paid to producers of $1 per gallon of milk is introduced. If there are no external costs and no external benefits, the marginal cost of the last gallon of milk produced is

A) $3.00 a gallon.
B) $3.50 a gallon.
C) $4.00 a gallon.
D) $4.50 a gallon.