What Is Implied When the Wage-Schooling Profile Is Drawn as a Concave

What is implied when the wage-schooling profile is drawn as a concave (i.e., increasing at a decreasing rate) function?

A) The marginal return to schooling increases as years of schooling increases.
B) The marginal return to schooling is positive but falling as years of schooling increases.
C) Average wages fall as years of schooling increases.
D) The cost of schooling increases but at a decreasing rate.
E) The cost of schooling decreases but at an increasing rate.