Which examples are categorized as variable costs?

Variable costs are expenses that change with the level of production or service delivery. They can fluctuate depending on the volume of work or materials needed. In this context, the correct choice refers to costs that vary based on usage or demand.

The elements classified in the correct option—such as call-back pay, complement, and magnetic beads—reflect costs that can vary significantly based on the specific needs of a project or service. For example, call-back pay may increase when there is a higher demand for staff during peak times. Complement, often associated with laboratory work involving assays or tests, varies based on the number of tests conducted. Magnetic beads are typically used in laboratory protocols, and their use will vary in quantity depending on the experiments being performed.

On the other hand, the other options include items that tend to be more fixed in nature or have a consistent cost regardless of changes in workload. For instance, items like typing trays and test tubes can be seen as fixed costs if they are purchased in bulk and used consistently over time. Similarly, items that are generally purchased regularly—like gloves, RPMI, and utilities—can have components that are variable, but in the context of laboratory operations, they often have fixed aspects too.