Over the entire range of output for which a firm's short-run average variable cost is decreasing, which of the
following must be true?
(A) Short-run marginal cost is falling.
(B) There are increasing returns to scale
(C) There are decreasing returns to scale
(D) Short-run marginal cost is less than short-run average variable cost.
(E) Short-run marginal cost is increasing due to diminishing returns. "