$4.$ A uniformly dense solid disk and a ring are joined by a massless bar. The disk and ring have the same radius and the same mass. The rotational inertia of the combined object is least relative to

$($a$)$ the midpoint of the bar

$($b$)$ a point to the left of the midpoint of the bar

$($c$)$ a point to the right of the midpoint of the bar

$($d$)$ a point on the object that depends on the object's motion

$($e$)$ the rotational inertia is the same relative to any point on the object