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

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

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