In the long run when a perfectly competitive firm experiences positive economic profits,

A) firms exit the industry, the market supply curve shifts rightward, and the market price falls.
B) firms enter the industry, the market supply curve shifts rightward, and the market price falls.
C) firms exit the industry, the market supply curve shifts leftward, and the market price rises.
D) firms enter the industry, the market supply curve shifts rightward, and the market price rises.