Following are five series of costs A through E measured at various volume levels. Identify each series as either fixed, variable, mixed, step-wise, or curvilinear.

Volume (Units)				Series A					Series B					Series C					Series D					Series E
	0				$	6,800				$	0				$	3,400				$	0				$	2,800
	400					6,800					4,320					4,000					6,900					2,800
	800					6,800					8,640					4,600					7,590					3,800
	1,200					6,800					12,960					5,200					8,280					3,800
	1,600					6,800					17,280					5,800					9,430					4,800
	2,000					6,800					21,600					6,400					11,040					4,800
	2,400					6,800					25,920					7,000					15,525					5,800