If variable cost of goods sold totaled $80,100 for the year (16,020 units at $5 each) and the planned variable cost of goods sold totaled $92,750 (13,250 units at $7 each), the effect of the unit cost factor on the change in contribution margin is:

a. $13,850 increase

b. $13,850 decrease

c. $32,040 decrease

d. $32,040 increase