答案：C
留意 $\theta = 210^\circ – 180^\circ = 30^\circ$。由此，該點的 $x$ 坐標
$\begin{array}{cl}
= & – 6 \cos 30^\circ \\
= & – 6 \times \dfrac{\sqrt{3}}{2} \\
= & -3\sqrt{3}
\end{array}$
及其 $y$ 坐標
$\begin{array}{cl}
= & -6 \sin 30^\circ \\
= & -6 \times \dfrac{1}{2} \\
= & -3
\end{array}$
所以，該點的坐標為 $(-3\sqrt{3},-3)$。
