If a Perfectly Competitive Industry Is in Long-Run Equilibrium, Which

If a perfectly competitive industry is in long-run equilibrium, which of the following is most likely to be true

A) some firms can be expected to leave the industry.
B) individual firms are not operating at the minimum points on their average total cost curves.
C) firms are earning a return on investment that is equal to their opportunity costs.
D) some factors are not receiving a return equal to their opportunity costs.