Three identical spheres, each of mass M, are placed at the corners of a right angle triangle with mutually perpendicular sides equal to 2 m (see figure). Taking the point of intersection of the two mutually perpendicular sides as the origin, find the position vector of centre of mass.

(1) ${4\over3}(\hat{i}+\hat{j})$

(2) $2(\hat{i}+\hat{j})$

(3) $(\hat{i}+\hat{j})$

(4) ${2\over3}(\hat{i}+\hat{j})$