Following are four series of costs measured at various volume levels. Identify each series as either fixed, variable, mixed, or step-wise. Hint: It can help to graph each cost series. Series D $ 5,000 Volume (Units) 8 400 800 1,200 1,600 2,090 2,400 Series A $0 3,600 7,200 10,800 14,480 18,000 21,600 Series B $ 2,500 3,100 3,700 4,300 4,900 5,500 6,100 Series C $ 1,800 1,000 2,000 2,000 3,080 5,000 5,098 5,898 5,000 5,000 5,080 3,000 4,999 Series A Series B Series C Series D