Which characteristic is true of the short run for both perfectly competitive firms and monopolists?

In the short run, both perfectly competitive firms and monopolists operate under the characteristic that firms cannot change the level of all inputs. This is because the short run is defined as a period during which at least one factor of production is fixed, typically capital. For instance, a firm might be able to vary its labor input (such as hiring more workers) or adjust variable costs, but it cannot change its plant size or other fixed costs.

This limitation on changing all inputs impacts how firms respond to market conditions—whether they are facing increased demand or higher costs. For both types of firms, the fixed nature of some inputs means that while they can adjust production levels to some extent, they are constrained by their existing capital and infrastructure.

The other characteristics do not apply universally to both perfectly competitive firms and monopolists in the short term. For example, perfectly competitive firms do have constraints regarding prices due to the nature of competition, while monopolists possess significant control over pricing, which is a defining feature of monopolistic market structures. Understanding these distinctions helps clarify why being unable to vary all inputs is a shared trait of both market types in the short run.