Which of the following is true of production possibility frontiers (PPFs)? 

a. The slope of the PPF reflects the opportunity costs of producing different combinations of two goods.

b. Combinations of goods can be produced inside or outside the PPF, ceteris paribus.

c. Points inside the PPF boundary are attainable and efficient.

d. Only points along the frontier line are attainable and efficient.

e. Both A and C

f. Both A and D