If the variable cost of goods sold totaled $90,000 for the year (18,000 units at $5.00 each) and the planned variable cost of goods sold totaled $86,400 (16,000 units at S5.40 each), the effect of the unit cost factor on the change in contribution margin is: Select one: a. $6,400 decrease b.$6,400 increase c. $7,200 increase d. $7,200 decrease