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

Isoquants

Curves that represent combinations of different inputs that produce the same output level, used in production theory.

Marginal Productivity

The increase in output resulting from a one-unit increase in the input of a particular resource, while holding inputs of all other resources constant.

Output Constant

A fixed amount of production that does not change with the levels of input.