Hubert has plans to go to a play and already has a $50 nonrefundable, nonexchangeable, and nontransferable ticket. Now Frances, whom Hubert has wanted to date for a long time, asks him to a party. Hubert would prefer to go to the party with Frances 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 Hubert decides to go to the party with Frances, 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.