The National Coal Association (NCA) is a group of independent coal-mining companies. Demand for coal falls, so the price drops. The Coal Refiners Association, a group of coal-refining companies, agrees to buy NCA’s coal and sell it according to a schedule that will increase the price. This agreement is

a. exempt from the antitrust laws.

b. subject to evaluation under the rule of reason.

c. a per se violation of the Sherman Act.

d. none of the above.