When a perfectly competitive industry is in long-run equilibrium,its firms:

A) earn more than zero economic profits.
B) combine their variable and fixed resources inefficiently.
C) are not in short-run equilibrium.
D) allocate all of their resources efficiently.

In long-run equilibrium, firms in a perfectly competitive industry allocate all their resources efficiently, meaning they produce at the lowest possible average cost. Since there are no barriers to entry or exit, competition drives economic profits to zero in the long run, so firms only earn normal profits. Therefore, choice D is correct.