Two spheres each of mass $M$ and radius $R / 2$ are connected with a massless rod of length $2 R$ as shown in the figure.
The moment of inertia of the system about an axis passing through the centre of one of the spheres and perpendicular to the rod is
(a) $\frac{21}{5} M R^{2}$
(b) $\frac{2}{5} M R^{2}$
(c) $\frac{5}{2} M R^{2}$
(d) $\frac{5}{21} M R^{2}$