opportunity costs

I'm glad my economics training has not completely gone to waste. Do you still remember?
via elia diodati:

You won a free ticket to see an Eric Clapton concert (which has no resale value). Bob Dylan is performing on the same night and is your next-best alternative activity. Tickets to see Dylan cost $40. On any given day, you would be willing to pay up to $50 to see Dylan. Assume there are no other costs of seeing either performer. Based on this information, what is the opportunity cost of seeing Eric Clapton?
A. $0
B. $10
C. $40
D. $50

If you notice a disturbing trend on this blog to link rather than write, I'm sorry. I'm tired.