Answer:
D. cannot be an f(x)=y function.
Step-by-step explanation:
An f(x) = y function has to have a specific property - there can be only one value for the same x - and the value has to be the same every time.
D. gives us all xs equal to 9, but with different ys - hence it cannot be an f(x) = y function (it could be an f(y) = x function though, but I assume that's not "allowed" under the problem statement).
B has two equal xs, but both times y=0, so it doesn't make it an invalid function.
