About which, axis be the moment of inertia be minimum?

The moment of inertia of a uniform cylinder of length l and radius R about its perpendicular bisector is I. What is the ratio (l)/( R) such that the moment of inertia is minimum ?

From a disc of radius R and mass M, a circular hole of diameter R, whose rim passes through the centre is cut. What is the moment of inertia of the remaining part of the disc about a perpendicular axis, passing through the centre?

From a circular disc of radius R and mass 9 M , a small disc of mass M and radius ( R)/(3) is removed concentrically. What will be the moment of inertia of the remaining disc about an axis perpendicular to the plane of the disc and passing through the centre?

A solid spherical ball and a hollow spherical ball of two different materials of densities rho_(1) and rho_(2) respectively have same outer radii and same mass. What will be the ratio of the moment of inertia ( about an axis passing through the centre ) of the hollow sphere to that of the solid sphere?

What is the CGS unit of moment of inertia ?