If variable cost of goods sold totaled $67,120 for the year (16,780 units at $4 each) and the planned variable cost of goods sold totaled $100,100 (14,300 units at $7 each), the effect of the unit cost factor on the change in contribution margin is: Oa $50,340 decrease Ob. $50,340 increase Oc. $9,920 increase Od. $9.920 decrease