Refer to Exhibit 7.4, which shows short-run average cost curves. Average cost is lowest for a medium plant when output is

c) between Qa and Qb

Explanation:

From the exhibit, the lowest cost will be recorded when output is between Qa and Qb because these points represent the lowest costs per unit for Curves A and C and the lowest points where output can be produced. Output being produced at costs lower than this is therefore the lowest for the medium plant.

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you have to put the parentheses like this:

in fact, you have to perform the operation inside parentheses first, and then proceed in order: