In which of the seven cases might monopoly arise?

■ An airline company cuts ticket prices to increase its market share.
■ A single firm controls a significant portion of a key resource needed to manufacture a medicine that has no close substitutes.
■ A barrier to entry exists, but the good has some close substitutes.
■ A firm offers discounts every Saturday.
■ A firm will lose most of its customers if it increases the price of its product.
■ The government issues an exclusive license to a state-owned firm to produce oil.
■ A power station experiences economies of scale even when it supplies electricity to all the residents in a city.