A retailer sells a product whose demand is stable and averages around 4000 per month. Each product costs $50 to purchase, and the order processing and shipping cost is $600 each time an order is placed. Each order takes 0.5 months to arrive. The retailer's interest rate is 5% per month. (a) What should the retailer's ordering policy be? Please specify the order quantity and timing. (10 points) (b) What is the optimal monthly cost? (5 points) (c) If the retailer would like to keep 500 units of safety stock, then what are the optimal order quantity and monthly cost? (5 points)