# A tetrahedron has vertices $O(0,0,0),\:A(1,2,1)\:B(2,1,3)\:C(-1,1,2)$. The angle between the faces $OAB$ and $ABC$ is ?
$(a)\:\:90^{\circ}\:\:\:\qquad\:\:(b)\:\:30^{\circ}\:\:\:\qquad\:\:(c)\:\:cos^{-1}(19/35)\:\:\:\qquad\:\:(d)\:\:cos^{-1}(17/31)$