Two masses each of mass $M$ are attached to the end of a rigid massless rod of length $L$. The moment of inertia of the system about an axis passing centre of mass and perpendicular to its length is
(a) $\frac{M L^{2}}{4}$
(b) $\frac{M L^{2}}{2}$
(c) $M L^{2}$
(d) $2 M L^{2}$