If variable cost of goods sold totaled Php90,000 for the year (18,000 units at Php5 each) and the planned variable cost of goods sold totaled Php88,000 (16,000 units at Php5.50 each), the effect of the quantity factor on the change in variable cost of goods sold is:

a. Php2,000 decrease

b. Php11,000 increase

c. Php9.000 increase

d. Php9,000 decrease