John's Friend Just Gave Him a Pair of Concert Tickets

John's friend just gave him a pair of concert tickets to see his favorite rock group perform this weekend. Each ticket sells for $25. John's boss asked him to work overtime the same weekend and at the same time as the concert. John currently makes $10.00 an hour and his overtime pay for the four hours his boss asked him to work is double the hourly rate. If John decides to go to the concert, his opportunity cost is

A) $80.00 in lost pay.
B) $40.00 in lost pay.
C) $50.00 because that is what his friend paid for the tickets.
D) $50.00 for the two tickets.
E) nothing because the concert tickets were free for John.