Cost ($) 200- 180 MC 160 140- 120 ATC- 100- AVC 8688 4 6 8 10 12 14 16 18 20 22 Q/t The graph shows cost functions for a firm (MC = marginal cost, ATC = average total cost, AVC = average variable cost). The average total cost of 18 units of output is O a) about $110 O b) impossible to estimate from the graph. O c) about $1,980 O d) about $90