The long-run average total cost curve is:

a.we cannot say without more detailed information on the firm's costs

b.steeper than the short-run average total cost curve as the firm incurs more fixed costs in the long run

c.steeper or flatter than the short-run average cost curve, but it depends on the long-run marginal cost curve

d.never steeper than short-run average total cost curves, as the firm can always choose the same input combinations as the short-run