Why Is a Production Possibilities Frontier Bowed Out (Concave)

Why is a production possibilities frontier bowed out (concave) ?

A) The bowed shape reflects increasing opportunity cost.
B) The bowed shape indicates that opportunity cost at first decreases at a decreasing rate, and then begins to decrease at an increasing rate.
C) The bowed shape indicates that opportunity cost at first increases at a decreasing rate, and then begins to increase at an increasing rate.
D) The bowed shape reflects constant opportunity cost.
E) The bowed shape reflects decreasing opportunity cost.