Help ASAP- I dont understand!!!

Is the relation below a function? If it is a function, state its domain.

{(0,2),(2,4),(1,7),(2,5)}

A. The relation is not a function.
B. The relation is a function. The domain is {0,1,2}
C. The relation is a function. The domain is {2,4,5,7}
D. The relation is a function. The domain is {0,1,2,4,5,7}