Which one of the following is an example of a sunk cost?
$1,500 of lost sales because an item was out of stock.
$1,200 paid to repair a machine last year.
$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.
$1,800 increase in comic book sales if a store ceases selling puzzles.