Concept:In production theory, the long run is a period when all factors of production become variable.
Explanation:The short run is characterised by at least one fixed factor of production.
In the long run, however, every input can be adjusted.
Firms may change capital, labour, land, and all other resources.
No factor remains fixed in the long run.
Thus, the key idea is that all factors become variable.
Options A and D describe conditions that are not true for the long run.
Option C is also incorrect because a firm can continue to exist in the long run.
Answer:B. all factors become variable.