For What Reason Are Production Possibilities Frontiers Usually Bowed Outward

For what reason are production possibilities frontiers usually bowed outward?

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 falls as one produces more and more of it.
C) It is because of the effects of technological change.
D) Resources are specialized, that is, some are better at producing particular goods rather than other goods.