The cost to produce Part A was $12 per unit in 2019. During 2020, it has increased to $15 per unit. In 2020, Sandhill Company has offered to supply Part A for $11 per unit. For the make-or-buy decision:

a) incremental revenues are $4 per unit.
b) net relevant costs are $3 per unit.
c) differential costs are $4 per unit.
d) incremental costs are $3 per unit.