Concept:The short run is a production period where some inputs cannot be changed.
Explanation:In the short run, the producer can adjust some inputs, such as raw materials or labour.
However, at least one input, usually capital or land, is fixed and cannot be varied.
This means not all inputs are fixed or variable during the short run.
Therefore, the correct condition is that at least one of the firm's inputs is fixed.
Answer:Option B. At least one of the firm's input is fixed.