AB Associates is reviewing its stock policy and has the following alternatives available for the evaluation of stock number 12789: (i) Purchase stock twice monthly - 100 units (ii) Purchase stock monthly - 200 units (iii) Purchase stock quarterly - 600 units (iv) Purchase stock biannually - 1,200 units (v) Purchase stock annually - 2,400 units It is ascertained that the purchase price per unit is $0.80 for deliveries up to 500 units. A 5% discount is offered by the supplier on the whole order where deliveries are 501 and up to 1,000 units, and 10% reduction on the total order for deliveries in excess of 1,000. Each purchase order incurs administration costs of $5.00. Storage, interest on capital and other costs are $0.25 per unit of average stock quantity held. (a) You are required to advise management on the optimum order size.