If $V(x, y)$ is the electric potential at a point $(x, y)$ in the $x y$-plane, then the level curves of $V$ are called equipotential curves because at all points on such a curve the electric potential is the same. Sketch some equipotential curves if $V(x, y)=c / \sqrt{r^2-x^2-y^2}$, where $c$ is a positive constant.