For the typical firm operating in the short run, the relationship between its marginal cost and average total cost curves is such that:

a) if marginal cost is greater than average total cost, then average total cost is falling.

b) if average total cost is greater than marginal cost, marginal cost must be falling.

c) if average total cost is less than marginal cost, average total cost must be rising.

d) if marginal cost equals average total cost, average total cost must be falling.