Three identical metal spheres on insulating stands are initi…

Three identical metal spheres on insulating stands are initially isolated from one another.  The spheres (A, B, and C) carry electric charges +12Q, -12Q, and +12Q respectively. Whenever two of the spheres come into contact with each other, each sphere ends up with the same net charge as the other sphere.  Spheres A and B are brought into contact with each other and separated. Spheres A and C are then brought into contact with each other and separated.  Which of the following correctly shows the final charge on each sphere?