Because the Marginal Product of Labor Measures the Quantity of Labor

Because the marginal product of labor measures the quantity of labor
Required to produce a unit of a good, the slope of the PPF can also be
Expressed as:

A) the ratio of abundance of labor to capital.
B) consumer utility.
C) the opportunity cost (in units of labor) to obtain an additional unit of good 1 in terms of what we give up of good 2.
D) the ratio of the marginal products of labor to the marginal product of capital.