11. Suppose that you must decide between attending a Taylor Swift concert or a Maroon 5 concert.

The concerts are at the same time on the same evening, so you cannot see both. You love Taylor Swift and would pay as much as $200 to see her perform. Tickets to her concert are $135. You are not as big a Maroon 5 fan, but a friend has just offered you a free ticket to the concert. If you decide to take the free ticket to see Maroon 5, what is your opportunity cost?