The position vectors of the points P and Q with respect to the origin O are a=i^+3j^−2k^ and b=3i^−j^−2k^ , respectively. If M is a point on PQ , such that OM is the bisector of PQ , then OM is
(1) 2(i^−j^+k^)
(2) 2i^+j^−2k^
(3) 2(−i^+j^−k^)
(4) 2(i^+j^+k^)