## Calculus (3rd Edition)

In cylinderical coordinates, we have \begin{aligned} &x=r \cos \theta\\ &y=r \sin \theta\\ &z=z \end{aligned} (a) When $\theta =0$, then $x=r, y=0$ and $z=z$. Hence, the statement is not correct. (b) When $\theta =0$, then $x=r, y=0$ and $z=z$. So the point lies on the $xz$-plane. Thus, the statement is correct.