Consider the Short-Run and Long-Run Cost Curves for a Firm

Consider the short-run and long-run cost curves for a firm.If factor prices decrease,

A) the firm will move down along both its long-run and short-run average cost curves.
B) the firm will move down along its long-run average cost curve only.
C) both the long-run and short-run average cost curves will shift downward.
D) there will be a downward shift in the long-run average cost curve but not in the short-run average cost curve.
E) there will be no change in the cost curves in the long run.