There are some passengers inside a stationary railway compartment. The track is frictionless. The centre of mass of the compartment itself (without the passengers) is $C_1$,while the centre of mass of the 'compartment plus passengers' system is $C_2$. If the passengers move about inside the compartment along the track,then:

• A
  both $C_1$ and $C_2$ will move with respect to the ground
• B
  neither $C_1$ nor $C_2$ will move with respect to the ground
• C
  $C_1$ will move but $C_2$ will be stationary with respect to the ground
• D
  $C_2$ will move but $C_1$ will be stationary with respect to the ground