Production Possibilities Frontiers Are Usually Bowed Outward

Production possibilities frontiers are usually bowed outward.This is because

A) the more resources a society uses to produce one good, the fewer resources it has available to produce another good.
B) it reflects the fact that the opportunity cost of producing a good decreases as more and more of that good is produced.
C) of the effects of technological change.
D) resources are specialized; that is, some are better at producing particular goods rather than other goods.