Answer: D. All of the above statements are true
Explanation:
Oligopolies exist in markets where supply is not saturated so they tend to form cartels where they can collude and charge a higher price to consumers so as to make more profits. Like a monopoly, this would lead to a deadweight loss because the urge to be competitive goes away and the market becomes socially inefficient.
In such a market, some firms will be tempted to break the cartel agreement and charge a lower price so as to gain market share. They stand a good chance of doing so in the short term but the other companies will react by reducing their prices as well which would reduce profits for the whole industry.