A firm faces a demand of 40,000 units per month for one of its lines of stock. The firms cost of holding stock is 2 per unit of average stock per month, and it costs the firm a total of 100 every time it orders and acquires a new shipment of stock. The firm is willing to face stock-outs and the cost of such is 6 per unit of average stock-out per month. Assuming the firm attempts to minimise overall inventory-related costs, how large (approximately) will the firm allow its stock-outs to become before each new stock delivery arrives?

a. 782 units b. 577 units c. 904 units d. 981 units