Which of the sets of ordered pairs represents a function?

A = {(3,-5), (4, 6), (-3, 9) , (2,7)}

B = {(2,4), (-1, -7), (5,6) , (4,3)}

Ebuka's monthly rent is $750. if ebuka pays the rent late, his landlord charges 4% interest per week that the payment is late. write a function that gives the total cost r(t), in dollars, of ebuka's rent if he pays it t weeks late.