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.