Which of the following is a function? A. (1,1),(-1, 1), (2,3).(-2,1))
B. {(1,1),(-1,-1),(-1,3), (2, 2)}
C. (0-1),(0,1),(1,0),(-1,2)
D.{(0,0), (1, 1), (0, -1), (1,-1)

Afitness center has 2 membership plans one has a low dollar sign up fee of 15 dollars and costs 38 dollars per month to join. a second plan has a higher sign up fee of 78 dollars but a lower monthly fee of 31 dollars for what number of days of the month is the coat the same