Dr$.$ Gatten began practicing dentistry on January $1,20$X$3.$ During January, she served $250$ patients who had their teeth examined and cleaned. In serving those patients, Dr$.$ Gatten spent money on $(1)$ supplies, including a $$clean teeth$$ bag that is given to each patient, $(2)$ the monthly salary for the dental hygienist, $(3)$ rent on the office, and $(4)$ payments on dental equipment. Of these costs, which is most likely to be a variable rather than a fixed cost?