All of the other options are the characterstics of the perfect
competition, the oligopoly firms exensively use the advertisement
and the sales promotion to gain the market power in the
economy.
When the private costs are exceed the social costs there is
negative externality and when the social cost exceed the private
cost there is positive goods. The public goods are the non rival
and non excludable in consumption so when the public uses the
public property they are only concerned with the private cost and
private benefit and not on the social cost and the social
benefit.
A tariff is generally imposed
on the imports of the country and the tariff makes an increase in
the price of the imported good.