On a production possibilities frontier, 500 pounds of apples and 1,200 pounds of bananas can be produced while at another point on the same frontier, 300 pounds of apples and 1,300 pounds of bananas can be produced. Between these points, what is the opportunity cost of producing a pound of bananas?
选项：

A:2 pounds of bananas
B:200 pounds of apples
C:2 pounds of apples
D:0.5 a pound of apples