Andrew decides to spend an hour playing tennis rather than working and earning a wage of $15 per hour. If playing tennis or working were the only two options Andrew had available, which of the following should be included in the opportunity cost of his decision?

Group of answer choices:

a) Nothing, because he enjoys playing tennis more than working

b) The increase in skill he obtains from playing tennis for that hour

c) The $15 he could have earned by working

d) The cost of his tennis racket which he purchased last year

Thank you!