Perfect competitors operate at peak efficiency in

A) both the short run and the long run.
B) neither the short run nor the long run.
C) only the short run.
D) only the long run.

Perfectly competitive firms operate at peak efficiency in the long run as they have enough time to adjust their inputs and production processes. In the short run, they may not have enough time to adjust and may operate at a suboptimal level until they reach the long run. Therefore, options A and C are incorrect, and option B is not a viable answer as perfect competition requires efficient operation in the long run.