Suppose That You Are Spending Two Hours a Day Studying

Suppose that you are spending two hours a day studying economics, and your grade is 85 per cent. You want a higher grade and decide to study for an extra hour a day. As a result, your grade rises to 90 per cent. Your marginal benefit is the

A) five point increase in your grade.
B) extra hour per day you spend studying.
C) five point increase in your grade minus the opportunity cost to you of spending the hour studying.
D) three hours per day you spend studying.