Two rings of the same radius and mass are placed such that their centers are at the same point and their planes are mutually perpendicular. The moment of inertia of this system about an axis passing through the center and perpendicular to the plane of one of the rings is (mass of ring = $m$,radius = $r$):

• A
  $ \frac{1}{2}mr^2 $
• B
  $ mr^2 $
• C
  $ \frac{3}{2}mr^2 $
• D
  $ 2mr^2 $