Eric Clapton versus Bob Dylan concert: The Question

A simple question taken from a very interesting book by Bob H Frank for everyone; doesn't matter whether you're an economist or not.

You've received a free promotional ticket to Eric Clapton concert which has no resale value. On the same night and at the same time, however, there is also a Bob Dylan concert. Bob Dylan is the only artist you'd ever consider going to listen to that night (apart from Eric Clapton, that is). On a normal day, you'd be willing to pay £50 for his ticket. The price of the Bob Dylan ticket for that night is actually £40.

Taken all of these information together, what is the opportunity cost - i.e. the cost of giving up the opportunity of doing one thing or consuming one thing and not the other - of going to see Eric Clapton concert? Is it:

A) £10

B) £40

C) £50

D) Zero.

Give yourself 5 minutes to think about this and post your answers in the comment below. No cheating now!

I'll reveal the answer in my next post...