Why is the marginal revenue curve for a perfectly competitive firm the same as its demand curve? Many plumbers charge the same price for coming to your house to fix a kitchen sink. Is this because...

Why is the marginal revenue curve for a perfectly competitive firm the same as its demand curve? Many plumbers charge the same price for coming to your house to fix a kitchen sink. Is this because plumbers are colluding? Do firms in a perfectly competitive market exhibit productive efficiency? Why or why not? Profit serves as both an incentive and a signal. Explain