A Problem Often Encountered When Oligopolists Try to Cooperate and Set

A problem often encountered when oligopolists try to cooperate and set output and prices in the market is that:

A) the number of firms in the industry is too small to form a successful cartel.
B) most oligopolists produce highly differentiated products.
C) some firms might cheat on the agreed-upon market price.
D) market demand and costs are inflexible and do not change.