Two identical particles are located at $\vec{x}$ and $\vec{y}$ with reference to the origin of three-dimensional coordinate system. The position vector of centre of mass of
the system is given by:
$\begin{array}{ll}\text { (a) } \vec{x}-\vec{y} & \text { (b) } \frac{\vec{x}+\vec{y}}{2}\end{array}$
(c) $(\vec{x}-\vec{y})$
(d) $\frac{\vec{x}-\vec{y}}{2}$