A firm's long-run average total cost curve would be horizontal if it experiences constant returns to scale at all output levels.
Constant returns to scale is a situation that occurs when the average cost does not change when there is an increase in output.
Constant returns to scale is a situation where an increase in input causes an equal proportionate increase in output level. Here the average cost of production will not change due to the increase in inputs.
Hence, a firm's long-run average total cost curve would be horizontal if it experiences constant returns to scale at all output levels.
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