Which Statement Is TRUE

Which statement 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 made.
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) For 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.