The table shows three short-run cost schedules for three plants of different sizes that a firm might build in the long run. Plant 1 Output 10 20 30 40 50 ATC $10 9 8 9 10 Plant 2 Output 10 20 30 40 50 ATC $15 10 7 10 14 Plant 3 Output 10 20 30 40 50 What is the long-run average cost of producing 20 units of output? ATC $ 20 15 10 8 9