Dr$.$ Gatten began practicing dentistry on January $1,20$X$3.$ During March, she served $300$ patients who had their teeth examined and cleaned. In serving these patients, she spent money on the following costs during March: $(1)$ 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. Which of these costs would be subtracted from revenues to arrive at contribution margin?

Supplies

Monthly salary for dental hygienist

Dental equipment payments

Rent on office