If economy uses inputs efficiently, then O a) MRTS = 1. O b) MRTS must be equal to the slope of a firm's isoquant curve. O c) the ratio of input marginal products must be equal to the ratio of input prices. O d) the quantity of one output cannot be increased without decreasing the quantity of the other. O e) every firm's MRTS is equal to every consumer's MRS. O f) every firm's MRTS is equal to the ratio of input prices. O g) MRTS = MRT. O h) outputs are on PPF