If the vertices of a triangle are $A(1,4,2),\:B(-2,1,2)\:C(2,-3,4)$ then the angle $B$ = ?

Question

$(a)\:cos^{-1}1/\sqrt 3\:\:\:\qquad\:(b)\:90^{\circ}\:\:\:\qquad\:(c)\:cos^{-1}\sqrt 3\:\:\:\qquad\:(d)\:cos^{-1}\sqrt 6/3$