Inventory decision, opportunity cost. A manufacturer of lawn mowers predicts that 240,000 spark plugs will have to be purchased during the next vear. The manufacturer estimates that 20,000 spark plugs will be required each month. A supplier quotes a price of S8 per spark plug. The supplier also offers a special discount option: If all 240,000 spark plugs are purchased at the start of the year, a discount of 9% off the S8 price will be given. The manufacturer can invest its cash at 8% per year Required