答案：C
$\begin{array}{rcl}
\pi r^2 \times \dfrac{120^\circ}{360^\circ} & = & 12 \pi \\
r^2 & = & 36 \\
r & = & 6
\end{array}$
$\begin{array}{rcl}
\pi r^2 \times \dfrac{120^\circ}{360^\circ} & = & 12 \pi \\
r^2 & = & 36 \\
r & = & 6
\end{array}$
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