Every week, Kate plays tennis for 2 hours, and her grade on each test is 70%. Last week, after playing for 2 hours, Kate considered playing for another hour. She decided to play for another hour, so she cut her study time by I hour. Her grade fell to 60%. (a) What was Kate's opportunity cost of the third hour of tennis? (b) Given that Kate played the 3rd hour, what can you conclude about her marginal benefit/ marginal cost of the 2nd hour of tenis?