Willis Company had a net operating income of $75,000 using variable costing and a net operating income of $57,000 using absorption costing. Variable production costs were $15 per unit. Total fixed manufacturing overhead was $120,000 and 10,000 units were produced. During the year, the inventory level: A)increased by 1,200 units.

B)increased by 1,500 units.

C)decreased by 1,500 units.

D)decreased by 1,200 units.

How do you get C as the answer??