How Does Oligopoly Differ from Perfect Competition and Monopolistic Competition

How does oligopoly differ from perfect competition and monopolistic competition?

A) Barriers to entry are lower in oligopoly industries than they are in perfectly competitive and monopolistically competitive industries.
B) Demand and marginal revenue curves are more useful for analysing oligopoly than they are for analysing perfect competition and monopolistic competition.
C) Oligopoly firms often react when other firms in their industry change their prices, it is difficult to know what the oligopolist's demand curve looks like.
D) The concentration ratios of oligopoly industries are lower than they are for perfectly competitive and monopolistically competitive firms.