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}