Four identical small spheres are attached to the corners of a light square frame of side L. For which of the following axes will the system have the smallest rotational inertia?
a) An axis perpendicular to the square and passing through the midpoint of one side
b) An axis lying in the plane of the square along one of its diagonals
c) An axis perpendicular to the square and passing through its center
d) An axis perpendicular to the square and passing through one corner