Which of the following statements about Production Possibility Frontiers (PPF) in a two good economy are always TRUE? l. A PPF is concave (bowed outward) when the opportunity cost of producing a good increases as more is produced ILA PPF is concave (bowed outward) when the opportunity cost of producing a good decreases as more is produced III. A PPF is straight when the opportunity cost of producing a good is 0 iv. A PPF is straight when the opportunity cost of producing a good is constant ' 0 ll., lil