In the Long Run in a Constant-Cost Industry

In the long run in a constant-cost industry:

A) A firm's supply curve is upward sloping but the industry supply curve is perfectly elastic at the minimum of ATC.
B) A firm's supply curve is upward sloping but the industry supply curve is perfectly elastic at the minimum of AVC.
C) Both the industry and a firm's supply curve are perfectly elastic at the minimum of ATC.
D) The supply curve for a firm is perfectly elastic at the minimum of average total cost and the industry supply curve is the horizontal summation of the upward sloping sections of the MC curves of firms in the industry (above the minimum of ATC) .
E) None of the above.