Which One of the Following Is an Example of a Sunk

Which one of the following is an example of a sunk cost?

A) $1,500 of lost sales because an item was out of stock
B) $1,200 paid to repair a machine last year
C) $20,000 project that must be forfeited if another project is accepted
D) $4,500 reduction in current shoe sales if a store commences selling sandals
E) $1,800 increase in comic book sales if a store commences selling puzzles