Concept:The law of diminishing returns operates only when at least one input is fixed while other inputs can be varied.
Explanation:In the short run, some inputs are variable and some inputs are fixed.
As more units of the variable input are added to the fixed input, the marginal product eventually falls.
This happens only in the short run because a fixed factor is present to limit production.
In the long run, all inputs are variable, so the law no longer applies in the same way.
Thus, the law of diminishing returns is a short run phenomenon because some inputs are variable and some remain fixed.
Answer:D. some inputs are variable