According to economists, one reason few professional athletes have phd’s is that the:
a. marginal cost of going to graduate school is too high.
b. explicit cost of going to graduate school would be too high.
c. comparative cost of going to graduate school is too high.
d. opportunity cost of going to graduate school is too high.
e. marginal benefit of going to graduate school is too high.

d. opportunity cost of going to graduate school is too high.

Explanation:

The cost of choosing an alternative instead of another is known as the opportunity cost. In order to obtain a PhD, athletes would have to delay their professional careers, which would cause then to potentially miss out on years of millionaire contracts. Therefore, the opportunity cost of going to graduate school is too high.

you have to put the parentheses like this:

in fact, you have to perform the operation inside parentheses first, and then proceed in order: