If variable cost of goods sold totaled $80,000 for the year (16,000 units at $5.00 each) and the planned variable cost of goods sold totaled $84,000 (15,000 units at $5.60 each), the effect of the unit cost factor on the change in variable cost of goods sold is:
a) $9,600 increase
b) $5,600 decrease
c) $9,600 Decrease
d) $5,600 increase