Which one of the following is the best example of a sunk cost?
a. Angelo's decided to accept project A rather than project B.
b. Burt's spent $18,000 last year patching potholes in its parking lot caused by a severe winter frost.
c. Lester's added desserts to its menu which caused its sandwich sales to increase.
d. A new fish sandwich was added to a menu and as a result hamburger sales declined.