Production Possibilities Frontiers Are Typically Concave (Bowed Out)from the Origin

Multiple Choice

Production possibilities frontiers are typically concave (bowed out) from the origin because

A) of the law of supply
B) there is usually a one-for-one trade-off in resources used in production
C) economies of scale enable firms to reduce the average costs of production as output rises
D) the opportunity cost of a good rises as the quantity of the good produced increases
E) resources are often left idle in the firm