Refer to Figure 2-4, Which of the following is not a result of the shift of the economy's production possibilities frontier from Graph (a) to Graph (b)? a. The opportunity cost of a cup of coffee is higher at all levels of coffee production. b. Production of four donuts and two cups of coffee becomes possible. c. Production of one donut and four cups of coffee becomes efficient. d. The trade-off between the production of donuts and coffee changes