A company that operated with a 30% average gross profit margin for a number of years had $100,000 of sales during the first quarter of this year. If it began the quarter with an $18,000 inventory at cost and purchased $72,000 of merchandise during the quarter, its estimated ending inventory by the gross profit method is:
A) $18,000
B) $20,000
C) $21,000
D) $30,000