Question

If net external force on a system of particles is zero, then match the following

Answer 1

$\left(A\right)$ Since net external force is zero. There will no any force acting on center of mass and its acceleration will be zero and constant.
$A\to 1,2$
$\left(B\right)$ Kinetic energy of system can vary but total mechanical energy is conserved therefore, KE may not be constant and may not be zero.
$B\to 3,4$
$\left(C\right)$ Since acceleration is zero. Therefore, velocity of center of mass is constant. and it is not necessarily zero.
$C\to 1,3$
$\left(D\right)$ Since KE can vary therefore, velocity of individual particle can vary and not necessarily zero.
Therefore,
$D\to 3,4$