Stephen Company produces a single product. Last year, the company had 20,000 units in its ending inventory. During the year, Stephen's variable production costs were $12 per unit. The fixed manufacturing overhead cost was $8 per unit in the beginning inventory. The company's net operating income for the year was $9,600 higher under variable costing than it was under absorption costing.

a)18,800 units

b) 19,200 units

c)19,520 units

d)21,200 units