The Difference Between Production Possibilities Frontiers That Are Bowed Out

The difference between production possibilities frontiers that are bowed out and those that are straight lines is that

A) bowed-out production possibilities frontiers apply to economies that face tradeoffs, whereas straight-line production possibilities frontiers apply to economies that do not face tradeoffs.
B) bowed-out production possibilities frontiers apply to economies in which resources are not specialized, whereas straight-line production possibilities frontiers apply to economies in which resources are specialized.
C) bowed-out production possibilities frontiers illustrate increasing opportunity cost, whereas straight-line production possibilities frontiers illustrate constant opportunity cost.
D) straight-line production possibilities frontiers illustrate real-world conditions, whereas bowed-out production possibilities frontiers illustrate more simplistic assumptions.