Please help me with these microeconomics questions:

18. Suppose that some firms in a perfectly competitive market are making positive economic profits. Which one of the following would not be

expected to occur?

(a) More firms would enter the market.

(b) All firms' economic profits would eventually be driven to zero at equilibrium.

(c) The supply curve will shift to the right.

(d) The equilibrium price will fall.

(e) The equilibrium quantity sold will fall.

19. A firm should continue to operate in the short-run as long as its revenues cover at least its . . .

(a) total costs.

(b) marginal costs.

(c) fixed costs.

(d) variable costs.

(e) transaction costs.

20. Which one of the following statements is true?

(a) Marginal cost is at its maximum where it intersects the average total cost curve.

(b) When marginal cost is greater than average total cost, the average total cost curve is decreasing.

(c) Marginal cost is always greater than average total cost but less than average variable cost.

(d) Average variable cost is at its minimum where it intersects the average total cost curve.

(e) Average total cost is at its minimum where it intersects the marginal cost curve.

21. In economic theory, purely competitive firms, monopolists, and monopolistically competitive firms all . . .

(a) set price equal to marginal cost to decide the level of output.

(b) maximize profits where marginal revenue equals marginal cost.

(c) face downward-sloping demand curves.

(d) are able to set prices, depending on what the market will bear.

(e) none of these.

22. A monopoly that arises because of economies of scale is referred to as a . . .

(a) local monopoly.

(b) natural monopoly.

(c) network monopoly.

(d) regulated monopoly.

(e) exclusionary monopoly.

23. Perfect price discrimination . . .

(a) results in lower prices for all consumers.

(b) maximizes deadweight loss.

(c) eliminates producer surplus.

(d) eliminates consumer surplus.

(e) reduces efficiency.