Which of the following relations are functions?
a. RO = {(1,7) (2,7) (4,7) (6, 7)}
b. RI = {(1, 2) (1, 3) (1, 4) (1,5)}
c. RI = {(x, y) (y, z) (z, t) (t, v)}
d. RD = {(x, y) (y, y) (z, t) (t, v)}​