The long run is a period when

Step 1: Analyze the question The question asks for the definition of the "long run" in economics, which relates to the theory of production and cost.

Step 2: Explain the concept Economists classify time periods by how flexible a firm's inputs are:

• Short run: at least one factor of production is fixed (for example, land, factory building, or heavy machinery). Output can only be changed by adjusting the variable factors, such as raw materials and labour.
• Long run: a period long enough for the firm to adjust all of its factors of production. Nothing is fixed. The firm can build a bigger factory, buy new machinery, or even leave the industry.

The long run is not a specific length of time like 5 years. It depends on the industry. A roadside baker may change all inputs within weeks, while an oil refinery may need many years.

Step 3: Evaluate the options

• A. Only the variable factors can be altered: This describes the short run, so it is wrong.
• B. All factors become variable: This is the correct definition of the long run.
• C. The firm will cease to exist: This is wrong. A firm may choose to exit, but the long run does not mean it ceases to exist.
• D. Only the fixed factors can be altered: This is wrong. It makes no sense because fixed factors cannot be altered in isolation.

Step 4: Answer The correct option is B. all factors become variable.

Tip: Remember "Long run = Lots of change." In the long run, all inputs are variable, so there are no fixed costs. In the short run, at least one input is stuck (fixed).