Select all statements that are true.

$$

f$($x$,$y$)=$x$2+$y$2$is a paraboloid while f$($x$,$y$)=$x$2+$y$2$is a cone.

$$

f$($x$,$y$)$ is a surface whereas f$($x$,$y$,$z$)$ is a family of surface or density of a solid

$$

level curves are traces along the z$-$axis