Which of the following statements are true regarding the velocity and acceleration of the center of mass of a system?
1) The velocity of the center of mass of a system is always zero.
2) The acceleration of the center of mass of a system is always zero.
3) The velocity of the center of mass of a system can be non-zero even if the system is at rest.
4) The acceleration of the center of mass of a system can be non-zero even if the system is at rest.