Final answer:
If a firm builds a larger plant and long-run average total cost does not change, it indicates constant returns to scale.
Step-by-step explanation:
If a firm builds a larger plant and long-run average total cost does not change, it indicates constant returns to scale.
Constant returns to scale occur when a firm increases its output or production scale while keeping the same proportion of inputs, resulting in no change in long-run average total cost.
Therefore, the statement 'If a firm builds a larger plant and long-run average total cost does not change, the firm has constant returns to scale' is True.