Among the people who are characterized below, who has the highest opportunity cost of leisure?
a. an attorney who earns $200 per hour and who plays golf during her leisure time
b. a medical doctor who earns $210 per hour and who sleeps during his leisure time
c. a retail clerk who earns $15 per hour and who watches tv during her leisure time
d. a waiter who earns $12 per hour and who reads poetry during his leisure time