Which of the following statements is true: (a)In the long run for all firms in a perfectly competitive constant-cost industry price equals average...

Which of the following statements is true:

(a)In the long run for all firms in a perfectly competitive constant-cost industry price equals average cost.

(b)In the long run for all firms in a perfectly competitive constant-cost industry price equals marginal cost.

(c)In the long run all firms in a perfectly competitive constant-cost industry earn zero economic profit.

(d)Only a and c are correct.

(e)a, b and c are correct.

Please help me with this question. What is the answer. Please provide working out and explanation please