During 2016, it cost Waterway Industries $28 per unit to produce part T5. During 2017, it has increased to $31 per unit. In 2017, Southside Company has offered to provide Part T5 for $24 per unit to Westa. As it pertains to the make-or-buy decision, which statement is true?

Differential costs are $7 per unit.

Incremental costs are $4 per unit.

Net relevant costs are $4 per unit.

Incremental revenues are $3 per unit.