At the end of last year, Gean Company had 30,000 units in its ending inventory. Gean’s variable production costs are $10 per unit and its fixed manufacturing overhead costs are $5 per unit every year. The company's net income for the year was $12,000 higher under variable costing than under absorption costing. Given these facts, the number of units of product in inventory at the beginning of the year must have been:

a. 28,800 units.
b. 27,600 units.
c. 32,400 units.
d. 42,000 units.