Which of the Following Statements Is Not Correct

Which of the following statements is not correct?

A) In a long-run equilibrium,firms must be operating at their efficient scale.
B) In the short run,the number of firms in an industry may be fixed.
C) In the long run,the number of firms can adjust to changing market conditions.
D) In the short run,firms must be operating at a level of output where price equals average variable cost.