In a Perfectly Competitive Market,the Short-Run Market Supply Curve Is

In a perfectly competitive market,the short-run market supply curve is

A) the horizontal sum of the supply curves of all the individual firms.
B) the vertical sum of the supply curves of all the individual firms.
C) vertical at the total level of output being produced by all firms.
D) horizontal at the current market price.
E) upward sloping with a constant slope.