Which of the Following Is True

Which of the following is true?

A) The long-run industry supply curve relates the price of a good or service to the quantity produced after all adjustments to a price change have been completed.
B) Every point on a long-run industry supply curve shows a price and quantity supplied at which firms in the industry are earning positive economic profit.
C) In establishing the long-run industry supply curve, factor costs and the number of firms are held constant.
D) In perfectly competitive industries, the long-run supply curve is always horizontal.