On the basis of the diagram above showing an economy's production possibilities curve for two goods, which of the following statements must be true?
I. The opportunity cost of moving from point P to point R is 10 units of Y.
II. The opportunity cost of moving from point R to point P is 8 units of X.
III. The opportunity cost of moving from point Q to point R is 0 units of Y.
Select one:
a. I, II, and III
b. I and II only
c. I only
d. III only
e. II and III only