Isoquants Are Downward Sloping Because

Multiple Choice

Isoquants are downward sloping because

A) as more units of an input are used to produce a product, total cost increases.
B) as more units of an input are used to produce a product, the firm's marginal productivity increases.
C) if more of one input is used, then less of the other input must be used to keep output constant.
D) Both B and C