For a natural monopoly, economies of scale

A) exist along the long-run average cost curve at least until it crosses the market demand curve.
B) and diseconomies of scale exist along the long-run average cost curve at least until it crosses the market demand curve.
C) lead to a legal barrier to entry.
D) as well as constant returns to scale and diseconomies of scale exist along the long-run average cost curve at least until it crosses the market demand curve.
E) are totally absent.

A