Every week, Ben plays tennis for one hour, and his grade on each econ test is 90 percent. Last week, after playing tennis for one hour, Ben considered playing for another hour. He decided to play for another hour and cut his study time by one hour. But last week, his econ grade fell to 80 percent. Given that Ben spent a second hour playing tennis, what can you conclude about the marginal benefit and marginal cost of the first hour of playing tennis