In microeconomics, the short run is defined as a time period in which at least one factor of production is fixed, typically capital such as premises, heavy machinery, or long term infrastructure. Within that same period, firms can usually adjust other inputs to respond to changes in output. Labour hours can be increased or reduced through overtime, shift patterns, or temporary staff, and raw material usage typically rises or falls directly with production volume. Costs that change with the level of output are variable costs. This is why labour and raw materials are generally treated as variable in the short run. Fixed costs, such as rent or certain salaried overheads, do not vary with output in the short run. Average costs are a per unit measure rather than a cost type, and marginal cost refers to the cost of producing one additional unit, which is influenced by variable inputs but is not itself the classification asked for here. The most appropriate answer is variable costs.