What Is the Law of Diminishing Returns

What is the law of diminishing returns?

A) As more of a fixed input is added to a variable input in the production process the resulting increase in output will,at some point,accelerate.
B) As more of a variable input is added to a fixed input in the production process the resulting increase in output will,at some point,accelerate.
C) As more of a variable input is added to a fixed input in the production process the resulting increase in output will,at some point,diminish.
D) As more of a variable input is added to a fixed input in the production process the resulting increase in output will,at some point,become negative.