When a Firm Operates at the Efficient Scale,what Explains the Characteristic

When a firm operates at the efficient scale,what explains the characteristic of the average-total-cost-curve

A) Its average revenue must be less than the minimum of average total cost.
B) Its average revenue must be equal to the minimum of average total cost.
C) The average-total-cost curve must be falling.
D) The average-total-cost curve must be rising.