Sage Company is operating at 90% of capacity and is currently purchasing a part used in its manufacturing operations for $18.00 per unit. The unit cost for the business to make the part is $21.00, including fixed costs, and $10.00, not including fixed costs. If 31,105 units of the part are normally purchased during the year but could be manufactured using unused capacity, the amount of differential cost increase or decrease from making the part rather than purchasing it would be a Oa. $93,315 cost increase Ob. $559,890 cost decrease Oc. $248,840 cost decrease Od. $248.840 cost increase