Sam Quit His Job as an Accountant with We Keep

Sam quit his job as an accountant with We Keep Books Accurately to open his own accounting firm. He earned $40,000 with the accounting firm We Keep Books Accurately. During the current year, Sam had revenues of $150,000 and total expenses of $110,000. For Sam, the opportunity cost of going into business was

A) $110,000.
B) $40,000.
C) $150,000.
D) zero, because he had a profitable business.