The main difference between the short run and the long run in economics is that:

• A. In the short run, all costs are variable, while in the long run all costs are fixed.
• B. In the short run, at least one input is fixed, while in the long run all inputs can be varied.
• C. In the short run, production is impossible, while in the long run production is possible.
• D. In the short run, firms can enter and exit the market freely, while in the long run they cannot.

Answer: (B)

Verified step by step guidance

1. Step 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 other inputs (like labor) can be varied.
2. Step 2: Recognize that in the long run, all inputs are variable, meaning firms can adjust all factors of production, including capital, labor, and technology, to optimize production.
3. Step 3: Analyze the given options by comparing them to these definitions. For example, the statement 'In the short run, all costs are variable' contradicts the concept that some inputs are fixed in the short run, so some costs are fixed.
4. Step 4: Evaluate the option stating 'In the short run, at least one input is fixed, while in the long run all inputs can be varied' as it aligns with the standard economic definitions of short run and long run.
5. Step 5: Confirm that other options are incorrect based on economic theory, such as the idea that production is impossible in the short run or that firms can freely enter and exit only in the short run, which is not accurate.