question_answer

A tetrahedron has vertices at 0 (0, 0, 0), A (1, 2, 1), B (2, 1, 3) and C\[\left( -1,\text{ }1,\text{ }2 \right)\] . Then the angle between the faces OAB and ABC will be

A)  \[{{\cos }^{-1}}\,\,\left( \frac{19}{35} \right)\]   

B)  \[{{\cos }^{-1}}\,\,\left( \frac{17}{31} \right)\]

C)  \[30{}^\circ \]                         

D)  \[90{}^\circ \]

E)  None of these