The Figure Above Shows the Production Possibilities Frontier for a Country

The figure above shows the production possibilities frontier for a country. The opportunity cost of a gallon of milk between combination point A and B is

A) 3 gallons of ice cream for a gallon of milk.
B) 1/3 of a gallon of ice cream for a gallon of milk.
C) 4 gallons of ice cream for a gallon of milk.
D) 1 gallon of ice cream for a gallon of milk.
E) zero because at point A zero milk is being produced.