Ali decides to attend the one-hour review session for microeconomics instead of working at his job. His job pays him $10 per hour. Ali's opportunity cost of attending the review session is:_______

a. the $10 he could have earned at his job.
b. the one-hour review session
c. equal to the benefit he gets from the review session.
d. the value of the session minus the $10 he could have earned at his job
e. nothing, because the review session does not cost anything.