In a competitive industry with constant costs, the long- run response to a change in market demand is
a. Firms earn positive economic profits, market output increases, each firm’s output increases
b. Firms earn positive economic profits, market output increases, each firm’s output remains constant
c. Firms earn zero economic profit, market output increases, each firm’s output remains constant
d. Firms earn zero economic profit, market output increases, each firm’s output increases.