What is the main difference between the short run and the long run (in economics)?
A. In the short run all inputs are fixed, while in the long run all inputs are variable. B. In the short run the firm varies all of its inputs to find the least-cost combination of inputs. C. In the short run, at least one of the firm s input levels is fixed. D. In the long run, the firm is making a constrained decision about how to use existing plant and equipment efficiently.