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

Which one of the following is an example of a sunk cost? Multiple Choice $1,800 increase in comic book sales if a store ceases selling puzzles $1,500 of lost sales because an item was out of stock $20,000 project that must be forfeited if another project is accepted $4,500 reduction in current shoe sales if a store commences selling sandals O $1.200 paid to repair a machine last year