The position vectors of the vertices A,B,C of a triangle are ^i−^j−3^k, 2^i+^j−2^k and −5^i+2^j−6^k respectively. The length of the bisector AD of the ∠BAC where D is on the line segment BC, is
a. 15/2
b. None of these
c. 1/4
d. 11/2