The first hour you study on Thursday night for your Friday morning Economics 101 final exam will improve your grade on the final exam from a score of 50 percent to a score of 70 percent. The next hour you study improves your exam grade to a score of 85 percent. The third hour of studying moves your grade up to a score of 90 percent. This is an example of what?

a. The marginal costs of studying

b. The marginal benefits of studying

c. The opportunity costs of studying

d. The sunk costs of studying