Mina decides to spend three hours working overtime rather than going to the park with friends. Because Mina earns $20 per hour for overtime work, Mina's opportunity cost of working is a. the $60 earned by working. b. the enjoyment Mina would have received by going to the park. c. nothing, since Mina would have received less than $60 worth of enjoyment from going to the park. d. the $60 minus the enjoyment Mina would have received from going to the park