A thin disc of mass M and radius R has mass per unit area $\sigma ( \mathrm { r } ) = \mathrm { kr } ^ { 2 }$ where r is the distance from its centre. Its moment inertia about an axis going through its centre of mass and perpendicular to its plane is:\\
(1) $\frac { M R ^ { 2 } } { 3 }$\\
(2) $\frac { M R ^ { 2 } } { 2 }$\\
(3) $\frac { M R ^ { 2 } } { 6 }$\\
(4) $\frac { 2 M R ^ { 2 } } { 3 }$