3. Which of the following statements is generally correct?

I. The long-run average total cost curve is U-shaped.

II. The short-run average total cost curve is U-shaped.

III. Firms tend to experience economies of scale at low levels of production and diseconomies of scale at high levels of production.

a. I only

b. II only

c. III only

d. I and II only

e. I, II, and III