Which relation represents a function?

I think its A 
yeah I believe its A

The answer is C. I think you have to start with a negative.

Ithe verbal model describing the above situation will be: a rental company that offers two rental plans, plan a and plan b, for the same economy size car. for both pans the total cost is given by, f(m) which is a function of the number of miles,m, that the car has been driven. plan a charges flat fee of $75  and an additional rate for an unlimited number of miles of 0.12 per mile. plan b has an offer of $ 0.35 per mile but with no flat fee charges for rental.ii. the rate of change for plan a will increase the the cost per mile by a factor of $0.12 while in plan b the rate of change will increase the cost per mile by a factor of $3.5.iii] if m=250 miles: the cost of plan a will be: f(250)=0.12×250+75=$105the cost of plan b will be: f(250)=0.35×250=$87.5comparing the cost of both plans, plan b is the most cost effective.