Which of the following statements about the PPF are correct? (Select all that apply.) a. When the economy operates on the PPF, it reaches its potential. b. When the economy moves between two points on the PPF, the economy reduces its production of one good to increase the production of the other: this defines efficiency, c. When the economy operates inside the PPF, there are idle resources, this defines inefficiency d. When resources are not allocated to their best use, the economy is inefficient