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举一反三
【单选题】设空间区域$\Omega =\left\{ (x,y,z)|\sqrt{{{x}^{2}}+{{y}^{2}}}\le z\le 1 \right\}$，则三重积分$\iiint\limits {\Omega }{\sqrt{{{x}^{2}}+{{y}^{2}}}\text{d}x\text{d}y\text{d}z}=$ ( ).
A.
$\text{ }\!\!\pi\!\!\text{ }$
B.
$\frac{\text{ }\!\!\pi\!\!\text{ }}{2}$
C.
$\frac{\text{ }\!\!\pi\!\!\text{ }}{3}$
D.
$\frac{\text{ }\!\!\pi\!\!\text{ }}{6}$
