What is the domain of the function in this table?

X | Y
1 | 2
2 | 4
3 | 3
4 | 2

CHIOCES:
A. (1, 2), (2, 4), (3, 3), 4,2)
B. {1,2,3,4}
C. (1,2)
D. {2,3,4}


PLEASE HELP URGENT