Last Year Christine Worked as a Consultant

Last year Christine worked as a consultant. She hired an administrative assistant for $15,000 per year and rented office space (utilities included) for $3,000 per month. Her total revenue for the year was $100,000. If Christine hadn't worked as a consultant, she would have worked at a real estate firm earning $40,000 a year. Christine's opportunity cost of working as a consultant last year was ________.

A) $15,000
B) $51,000
C) $40,000
D) $36,000