Concept:In the long run, a firm can change the quantity of all factors of production used.
Explanation:In the short run, at least one factor, usually capital, is fixed.
In the long run, however, there is enough time to adjust every factor.
This means all inputs, such as land, labour, capital, and entrepreneurship, can be varied.
Therefore, all factors of production are considered to be variable in the long run.
Answer:C. variable