Which of the following statements about marginal and average cost is correct?

a. If marginal cost exceeds average cost, average cost may be rising or falling.

b. If marginal cost is rising, average cost must be rising.

c. If marginal cost is less than average cost, average cost must be falling.

d. If average cost is falling, marginal cost must be falling.

e. Marginal cost can never fall below minimum average cost.