A man of mass `80 kg` stands on a plank of mass `40 kg`. The plank is lying on a smooth horizontal floor. Initianlly both are at rest. The man starts walking on the plank towards north and stops after moving a distance of `6 m` on the plank. Then 1. The centre of mass of plank-man system remains stationary 2. The plank will slide to the north by a dsitance `4m` 3. The plank will slide to the south by a distance `4 m` 4. The plank will slide to the south by a distance `12 m`