Internal forces acting within a system of particles can alter

Why do internal forces acting within a system not affect its motion?

$Assertion$ : 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.

$A$ rope ladder with a length $l$ carrying a man with a mass $m$ at its end is attached to the basket of a balloon with a mass $M$. The entire system is in equilibrium in the air. As the man climbs up to the end of the ladder into the balloon,the balloon descends by a height $h$. Then the potential energy of the man:

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.

Two spheres of masses $2M$ and $M$ are initially at rest at a distance $R$ apart. Due to mutual force of attraction,they approach each other. When they are at separation $R/2$,the acceleration of the centre of mass of the system would be .......