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?

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

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

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

4) Denise pays $6 each time she visits the gym.

Question

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? 

1) Denise pays an annual fee for an all-inclusive gym membership. 

2) Denise pays an annual fee for a gym membership and $15 each time she chooses to take a spin class. 

3) Denise pays $50 per month for 10 monthly visits, then $3 for each additional visit. 

4) Denise pays $6 each time she visits the gym.