The Average Total Cost Curve

The average total cost curve:

A) slopes downward as long as the marginal cost is greater than the average total cost.
B) slopes downward as long as the marginal cost is less than the average total cost.
C) is the sum of the marginal, variable, and fixed cost curves.
D) slopes upward as long as the marginal cost is less than the average total cost.
E) is unaffected by the concept of diminishing returns.