Three identical spheres,each of mass $M$,are placed at the corners of a right-angled 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 the center of mass.

• A
  $\frac{4}{3}(\hat{i}+\hat{j})$
• B
  $2(\hat{i}+\hat{j})$
• C
  $(\hat{i}+\hat{j})$
• D
  $\frac{2}{3}(\hat{i}+\hat{j})$