Some factors of production cannot be changed in the short run. If the firm decides to upgrade its technology, construct a new building, or expand its machinery and equipment capacity today, it would take quite some time to realize any of these changes. Therefore, a period in which certain inputs cannot be changed is called the short run. These inputs are fixed in the short run. Therefore, they are called fixed inputs. The firm, however, can change certain inputs rather quickly—in the short run— and these inputs are called variable inputs. Labor is a typical variable input. The correct answer is D.