An operations planner, Lovell Bradley, is developing a sales and operations plan that involves back orders. The companys demand and production rates for the next four periods are as follows.

Period	Demand	Production
1	8,200	8,000
2	10,000	8,000
3	7,000	8,000
4	6,600	7,600

Beginning inventory at the start of period 1 is 400 units.

a) The company holds a positive level of ending inventory in periods [ Select ] ["1 and 4", "2 and 4", "1 and 2", "3 and 4", "1 and 3", "2 and 3"] , and this level equals [ Select ] ["800", "200", "400", "1800"] in each of the periods.

b) The company has to place back orders in periods [ Select ] ["2 and 4", "1 and 3", "1 and 4", "2 and 3", "1 and 2", "3 and 4"] , and the quantities of the back orders are [ Select ] ["1800", "400", "800", "200"] and [ Select ] ["800", "1800", "200", "400"] , respectively.