The manager at Frame Manufacturing reported the cost to product Frame A was $$22$ per unit in $2011$ and in $2012$ the cost increased to $$24$ per unit. In $2013,$ the manager at a local supplier offered to supply Frame A for $$20$ per unit. For the make$-$or$-$buy decision:

Group of answer choices

incremental revenues are $$4$ per unit.

incremental costs are $$2$ per unit.

net relevant costs are $$2$ per unit.

differential costs are $$4$ per unit.

None of these are correct.