Mary must decide whether to increase her college course load from four to five courses next semester. Using marginal analysis and assuming that Mary is rational, she should add the fifth course
A. only if the marginal cost of doing so is zero
B. If its marginal benefit is positive
C. If the marginal benefit of the course exceeds its marginal cost to her
D. If its marginal cost to her exceeds the marginal benefit she will get form it
Answer: C