A retailer uses a perpetual inventory system. Compute the firm's end-of-month inventory at cost, if monthly sales (at cost) = $600,000; monthly sales (at retail) = $750,000; monthly purchases (at cost) = $250,000; and beginning inventory (at cost) = $600,000.

a. $75,000
b. $150,000
c. $250,000
d. $400,000