Plant sizes get larger as you move from ATC-1 to ATC-4.

Output ATC-1 ATC-2 ATC-3 ATC-4 1,500 $ 10 15 $ 20 $ 30 2,000 8 12 17 25 2,500 9 10 15 20 3,000 12 8 13 18 3,500 15 6 11 16 4,000 18 10 9 14 4,500 20 12 7 12 5,000 24 15 11 10 5,500 29 19 13 8 6,000 35 25 15 9

Which plant size would produce at the least cost for the 3,000-4,000 range of output?