which of the following relations describes a function?
a. {(1,0), (1,2), (2,1), (2,2)}
b. {(5,2), (5,3), (6,2), (6,3)}
c. {(-2,-1), (-2,1), (-3,-1), (-3, 1)}
d. {(-2, -3), (-3,-2), (2,3), (3,2)}