Which of the following is correct?

a.In perfect competition, most firms in an industry are large.

b.In a monopoly, the firm that is the only supplier does not have to worry about how much consumer demand will fall as it raises its prices.

c.All things considered, perfect competition is the ideal situation.

d.In monopolistic competition, there are fewer sellers than in perfect competition but still many buyers.

e.In order for an oligopoly to exist, there must not be more than three sellers.