Jack has a ticket to see Bo Bice for which he paid $30 yesterday. He takes an unpaid day off from work to get ready for the concert. When he arrives at the concert, five different people offer him $70 for his ticket. Jack decides to keep his ticket. At the time he makes this decision, his opportunity cost of seeing Bo Bice is:
A.$70 plus his forgone earnings.
B.$70.
C.$40.
D.$30 plus his forgone earnings.
正确答案:$70.