Answer:
the $4,000 spent on a new software
Explanation:
Sunk costs are costs that the business has already incurred and they cannot be recovered no matter what decision they make.
in this case, the company must decide whether to hire 2 part time employees or only on full time employee. But no matter which decision they make, the money spent on the new computer software cannot be recovered. That is why the money spent on the new software is considered a sunk cost.