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

( A ) $\cos^{-1} (\frac{19}{35})$
( B ) $30^{\circ}$
( C ) $90^{\circ}$
( D ) $\cos^{-1} (\frac{17}{31})$