Darnell has plans to go to a play and already has a $50 nonrefundable, nonexchangeable, and nontransferable ticket. Now Vicky, whom Darnell has wanted to date for a long time, asks him to a concert. Darnell would prefer to go to the concert with Vicky and forgo the play, but he doesn't want to waste the $50 he spent on the play ticket.

From the perspective of an economist, if Darnell decides to go to the party with Vicky, what has he just done?

a. Incorrectly allowed a sunk cost to influence his decision
b. Made a choice that was not optimal
c. Correctly ignored a sunk cost