Why is the answer A? How would I go about solving this question?

4. Every time a particular retailer places an order from a supplier in another country, the retailer incurs a fixed fee of $100 (for shipping and customs charges). The purchase cost per item is $1.50 and the holding cost for one item for one year would be 40% of this cost. The weekly demand at the retailer is 60 units. Assume 50 weeks a year in the following. What is the ordering quantity each time? A. 1000 B. 1500 C. 2000 D. 3600 E. None of the above A