Volume(units) Series 1 Series 2 Series 3 Series 4 0 $450 $0 $800 $100 100 450 800 800 105 200 450 1,600 800 120 300 450 2,400 1,600 145 400 450 3,200 1,600 190 500 450 4,000 2,400 250 600 450 4,800 2,400 320 Listed here are four series of separate costs measured at various volume levels. 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.)