答案：B
設 $x\text{ cm}$ 為該平截頭體上底的半徑。由此，可得
$\begin{array}{rcl}
\dfrac{4}{12} & = & \dfrac{x}{9} \\
\dfrac{1}{3} & = & \dfrac{x}{9} \\
x & = & 3 \\
\end{array}$
所以，該平截頭體的體積
$\begin{array}{cl}
= & \dfrac{1}{3} \pi (9)^2 (12) – \dfrac{1}{3} \pi (3)^2 (4) \\
= & 312 \pi \text{ cm}^2
\end{array}$
