Why does the NPV of a college education tend to be positive but relatively small?

A) College students tend to be young and poor, so their opportunity cost of capital is high.
B) College graduates typically have limited on-the-job skills, so their initial returns from the education investment tend to be small.
C) There are few barriers to entry for a college education.
D) Most of these studies are based on nominal interest rates, so the NPV is smaller than we would achieve under real interest rates.

C