Sage Company is operating at 90% of capacity and is currently purchasing a part used in its manufacturing operations for $13.00 per unit. The unit cost for the business to make the part is $21.00, including fixed costs, and $12.00, excluding fixed costs. If 39,230 units of the part are normally purchased during the year but could be manufactured using unused capacity, what would be the amount of differential cost increase or decrease from making the part rather than purchasing it? a. $39,230 cost increase b. $39,230 cost decrease c. $313,840 cost increase d. $509,990 cost decrease