Denise decides to spend three hours working overtime rather than watching a video with her friends. She earns $10 an hour. Her opportunity cost of working is
a. the $30 she earns working.
b. the $30 minus the enjoyment she would have received from watching the video.
c. the enjoyment she would have received had she watched the video.
d. nothing, since she would have received less than $30 of enjoyment from the video.