Which of the following statements is true about the graph of a production possibilities curve? a. The cost of producing watermelons and shoes is roughly even on the production possibilities frontier. b. It costs 2 million more tons of watermelons to produce 14 million tons of shoes. c. The cost of moving from producing no watermelons to producing 8 million tons of watermelons is 1 million pairs of shoes. d. One million tons of watermelons must be sacrificed to produce 8 million pairs of shoes.