Which of the following surfaces represent graphs of fuctions, and which ones don't?

This image is also a link, so you can click on it to see interactive graphs. Feel free to rotate each plot. When in doubt, look at the various contour lines $-$ they can help you answering the question.

Important:

There is no point $(,,)$

of the surface in Plot $4$ for which $=0$

$.$

There is no point $(,,)$

of the Plot $6$ for which $==0$