Four particles, each of mass 1kg, are placed at  the corners of a square OABC of side 1m. ‘O’ is at the origin of the coordinate system. OA and OC are aligned along positive x-axis and   positive y-axis respectively. The position vector  of the centre of mass is (in ‘m’)
 

• A

  $\overrightarrow{i} +\overrightarrow{j}$

• B

  $( \overrightarrow{i}+\overrightarrow{j})/2$

• C

  $\overrightarrow{i}-\overrightarrow{j}$

• D

  $\frac{\overrightarrow{i}-\overrightarrow{j}}{2}$