When a Perfectly Competitive Industry Is in Long-Run Equilibrium, All

When a perfectly competitive industry is in long-run equilibrium, all firms in the industry

A) earn zero economic profits
B) produce a level of output where short-run marginal cost is equal to short-run average total cost
C) produce a level of output where long-run marginal cost is equal to long-run average cost
D) All of the above are correct