In the short run, A. at least one of the firm's inputs is fixed, while in the long run, the firm is able to vary all its inputs, adopt new technology, and change the size of its physical plant. B. all of the firm's inputs are fixed, while in the long run, the firm is able to vary all its inputs, adopt new technology, and change the size of its physical plant. C. all of the firm's inputs are variable, while in the long run, the firm is able to vary all its inputs as well as adopt new technology and change the size of its physical plant. D. at least one of the firm's inputs is fixed, while in the long run, the firm is either able to vary all its inputs, adopt new technology, or change the size of its physical plant. E. at least one of the firm's inputs is fixed, while in the long run, at least one of the firm's inputs is variable.

The short run is a notion that within a certain period of time, one or more input is fixed while the remaining inputs are variable. It shows that the way an economy behaves depends on the duration of time it has to acknowledge the changes. There is a key principle regarding the short and long run that states that firms face both variable and fixed costs in the short run while in the long run, all inputs are variable.

In the short run, contracts, leases, and wage agreements hinders the ability of a firm to adjust its production or wages in order to maintain its profit rate. There are no fixed costs in the long run.