The basic difference between the short run and the long run in microeconomics is that:

• A. In the short run, firms cannot make any production decisions, while in the long run they can.
• B. In the short run, firms always earn zero economic profit, while in the long run they earn positive economic profit.
• C. In the short run, at least one input is fixed, while in the long run all inputs can be varied.
• D. In the short run, all costs are variable, while in the long run all costs are fixed.

Answer: (C)

Verified step by step guidance

1. Understand the definitions of the short run and the long run in microeconomics: The short run is a period during which at least one input (such as capital or land) is fixed and cannot be changed, while the long run is a period long enough for all inputs to be varied.
2. Recognize that in the short run, firms can adjust some inputs (like labor) but cannot change fixed inputs, which limits their production decisions to varying only variable inputs.
3. In the long run, firms have the flexibility to adjust all inputs, including those that were fixed in the short run, allowing them to fully optimize production and costs.
4. Evaluate the given options by comparing them to these definitions: the correct distinction focuses on input flexibility, not on profit levels or cost classifications.
5. Conclude that the correct statement is: 'In the short run, at least one input is fixed, while in the long run all inputs can be varied,' because this accurately captures the fundamental difference between the two time frames in production theory.