Last year, Brinkman Company had 20,000 units in its ending inventory. During the year, Brinkman'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 income for the year was $9,600 higher under variable costing than it was under absorption costing. Given these facts, the number of units of product in the beginning inventory last year must have been:
a. 21,200.
b. 19,200.
c. 18,800.
d. 19,520.