behaves as a variable cost is:

A$.$ the wages of an assembly worker who is paid according to a piece rate system.

B$.$ depreciation expense that is based on the number of units produced.

C$.$ the salary of a production supervisor who is paid the same salary regardless of the level of production

D$.$ the wages of factory workers who are paid based on the number of hours they work, but who do not go home when the factory is idle.