In a system of particles,internal forces can change (for the system)

State Newton's second law for a system of particles.

Choose the correct statement$(s)$:
$(A)$ The position of the centre of mass of a system is dependent on the choice of coordinate system.
$(B)$ Newton's second law of motion is applicable to the centre of mass of the system.
$(C)$ Internal forces cannot change the state of the centre of mass.
$(D)$ Internal forces can change the state of the centre of mass.

The multiplication of the total mass of a system and the acceleration of its centre of mass denotes what?

$Assertion$ : If no external force acts on a system of particles,then the centre of mass will not move in any direction.
$Reason$ : If net external force is zero,then the linear momentum of the system changes.

Assertion $(A):$ If there is no external torque on a body about its centre of mass,then the velocity of the centre of mass remains constant.
Reason $(R):$ The linear momentum of an isolated system remains constant.