Step-by-step explanation:
If in any set, you can find 2 or more points that lie on the same vertical,
then it is NOT a function, that is, the same value of x yields 2 or more different
values of y, you don't have a function.
e.g. D is definitely not a function. You have (-2,4) and (-2,-4) , the same x of -2 giving you a y value of 4 and -4.