If we assume only one factor (labor), we can demonstrate on the PPF the opportunity cost of producing less of one good and more of the other good by:
a. taking the sum of the marginal products of labor for the two goods.
b. taking the difference of the marginal products of labor for the two goods.
c. taking the ratio of the marginal products of labor for the two goods times –1.
d. taking the average of the marginal products of labor for the two goods.