A tetrahedron has vertices at $O(0,0,0), A(1,2,1) B(2,1,3)$ and $C(-1,1,2).$ Then the angle between the faces $OAB$ and $ABC$ will be :

Question

${90^ \circ }$

${\cos ^{ - 1}}\left( {{{19} \over {35}}} \right)$

${\cos ^{ - 1}}\left( {{{17} \over {31}}} \right)$

${30^ \circ }$

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