What does the principle of diminishing returns state?
a. In the short run, at least one input is fixed.
b. In the long run, fixed inputs are added to variable inputs.
c. As more units of a variable input are added to another set of variable inputs, output will increase or diminish, depending on the ratio of the input sets.
d. As more units of a variable input are added to a set of fixed inputs, the resulting additions to output eventually become increasingly smaller.