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Assume a tetrahedron is formed by the coordinates planes and the plane $x+y+z=1$ with constant density, find its center of mass.

$(\cfrac{1}{2},\cfrac{1}{2},\cfrac{1}{2})$

$(\cfrac{1}{4},\cfrac{1}{4},\cfrac{1}{4})$

$(\cfrac{1}{2}, \cfrac{1}{4}, \cfrac{1}{2})$

$(\cfrac{1}{4},\cfrac{1}{2},\cfrac{1}{4})$

None of the above