答案：B
$\begin{array}{rcl}
(n-2)\times 180^\circ & = & 3240^\circ \ \text{(多邊形的內角和)} \\
n-2 & = & 18 \\
n & = & 20
\end{array}$
$\begin{array}{rcl}
(n-2)\times 180^\circ & = & 3240^\circ \ \text{(多邊形的內角和)} \\
n-2 & = & 18 \\
n & = & 20
\end{array}$
由此，該正多邊形的外角
$\begin{array}{cl}
= & \dfrac{360^\circ}{20} \\
= & 18^\circ
\end{array}$
