Listed here are four series of separate costs measured at various volume levels. Volume (Units) Series 1 Series 2 Series 3 Series 4 0 $ 0 $ 450 $ 100 $ 800 100 800 450 105 800 200 1,600 450 120 800 300 2,400 450 145 1,600 400 3,200 450 190 1,600 500 4,000 450 250 2,400 600 4,800 450 320 2,400 Examine each series and identify whether it is best described as a fixed, variable, step-wise, or curvilinear cost. It can help to graph the cost series. Series 1 Series 2 Series 3 Series 4