When finding if a set is a function, the best method is looking at the x coordinates. Make sure that the if the x coordinates repeats that the y coordinates are identical, meaning if a point repeats; (2,0) and (2,0). If the x coordinates repeats itself but has two different y coordinates, it is not a function: (2,0) and (2,6).
A. x=0 repeats with two different y coordinates, NOT FUNCTION
B. No x's repeat, FUNCTION
C. x=2 repeats with two different y coordinates, NOT FUNCTION
D. x=6 repeats with two different y coordinates, NOT FUNCTION