If variable cost of goods sold totaled $66,440 for the year (16,610 units at $4 each) and the planned variable cost of goods sold totaled $99,330 (14,190 units at $7 each), the effect of the unit cost factor on the change in contribution margin is:

a.$9,680 increase

b.$49,830 decrease

c.$9,680 decrease

d.$49,830 increase