A Short-Run Average Total Cost Curve and a Long-Run Average

Multiple Choice

A short-run average total cost curve and a long-run average cost curve are tangent

A) where the short-run cost curve is downward sloping.
B) where the short-run cost curve is upward sloping.
C) when the plant size is at the optimal level for that level of output.
D) where the short-run cost curve is downward sloping and the plant size is optimal.
E) by coincidence.