With reference to figure of a cube of edge $a$ and mass $m_{1}$ which of the following is the incorrect statement?
(O is the centre of the cube)
(a) The moment of inertia of cube about $z^{\prime}$ is
$$
I_{z}^{\prime}=I_{z}+\frac{m a^{2}}{2}
$$
(b) The moment of inertia of cube about $z^{\prime \prime}$ is
$I_{z}^{\prime \prime}=I_{z}+\frac{m a^{2}}{2}$
(c) $I_{x}=I_{y}$
(d) None of these