Consumers of Laundry Detergent View Brands as Different and So

Consumers of laundry detergent view brands as different and so each brand effectively has some market power.Prices are set much higher than marginal cost,yet detergent firms earn a normal rate of return.
a.Use a diagram with demand for the output of a single firm,marginal cost and average cost curves to show a monopolistically competitive firm in equilibrium.
b.Why do firms just earn a normal rate of return? That is,why do firms in this market earn zero profits?
c.Suppose that each firm in this industry was required to pay a fixed annual fee to operate (a tax that does not vary with the output of the firm).What would this tax do to the number of firms and the price each firm charges? (Using a new graph may help your argument. )