What is the domain of the given function? {(3, -2), (6, 1), (-1, 4), (5, 9), (-4, 0)}
a. {x | x = -4, -1, 3, 5, 6}
b. {y | y = -2, 0, 1, 4, 9}
c. {x | x = -4, -2, -1, 0, 1, 3, 4, 5, 6, 9}
d. {y | y = -4, -2, -1, 0, 1, 3, 4, 5, 6, 9}