The cost to produce Part A was $$33$ per unit in $2021.$ During $2022,$ it increased to $$36$ per unit. In $2022,$ Cullumber Company has offered to supply Part A for $$27$ per unit. No fixed costs are avoidable. For the make$-$or$-$buy decision,

incremental costs are $$3$ per unit.

net relevant costs are $$3$ per unit.

differential costs are $$9$ per unit.

incremental revenues are $$9$ per unit.