Let ∏ denote the xy-plane. Which of the following vectors are normal to ∏?
\text { (a) } n _{1}=\left[\begin{array}{l}2 \\1 \\3\end{array}\right]
\text { (b) } {n}_{2}=\left[\begin{array}{l}3 \\2 \\0\end{array}\right]
\text { (c) } n _{3}=\left[\begin{array}{l}0 \\0 \\5\end{array}\right]