Morton Salt Co. gave quantity discounts to some purchasers of its Blue Label brand of salt within the same market. It charged three different prices for the salt, depending on the quantity purchased. Distributors that purchased more than 50,000 cases were charged $1.35 per case, while distributors that purchased fewer than 50,000 cases were charged between $1.40 to $1.60, depending on how much salt they purchased. The Federal Trade Commission ordered Morton Salt to cease and desist these quantity discounts, and Morton appealed. The case ultimately ended up before the United States Supreme Court. The Court most likely ruled that:

a. The quantity discounts were an illegal exclusionary practice under the Sherman Act

b. The quantity discounts involved illegal predatory pricing under the Clayton Act

c. The quantity discounts were legal under all U.S. antitrust laws

d. The quantity discounts involved illegal price discrimination under the Robinson-Patman Act