Which of the Following Characteristics Make an Industry More Conducive

Which of the following characteristics make an industry more conducive to collusive behavior?

A) Firms in the industry have very different marginal costs of production.
B) Firms in the industry produce goods with significantly different product attributes.
C) Firms in the industry are operating at a maximum productive capacity that cannot be easily altered in the short run.
D) Firms in the industry serve customers that can easily switch between firms as they search for the best price.