The Standard economic suppositions recommend that the production possibilities frontiers are concave from beneath [from origin] mainly because: (i) People desire additional units of a good less the more of good they encompass. (ii) The relative productivity of various resources differs for various kinds of goods. (iii) Optimal decisions need equating marginal costs with marginal advantages. (iv) Production costs tend to drop as more and more of good is generated.
What is the right answer?