In cylindrical coordinate system, the potential produced by a uniform ring charge is given by $\phi = f(r, z)$ where $f$ is a continuous function of $r$ and $z$. Let $\overrightarrow{E}$ be the resulting electric field. Then the magnitude of $\nabla \times \overrightarrow{E}$
- Increases with $r$
- Is $0$.
- Is $3$.
- Decreases with $z$.