Which sentence represents a relationship between the number of visits
to the gym that Denise makes and her total gym cost that results in a
line with zero slope? 

A. Denise pays an annual fee for an all-inclusive gym membership.

B. Denise pays an annual fee for a gym membership and $15 each time she chooses to take a spin class.

C. Denise pays $50 per month for 10 monthly visits, then $3 for each additional visit.

D.Denise pays $6 each time she visits the gym.