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