If x, y∈{0, 1, 2, 3, 4, 5} Then, which of the following are the function in the given set/
(a) f1 = {(x, y) : y = x - 1}
(b) f2 = {(x, y) : x + y < 3}
(c) f3 = {(x, y) : x > y}
(d) f4 = {(x, y) : x + y = 4}