In the law of conservation of linear momentum,which two physical quantities remain constant,and which physical quantity remains zero?

(N/A) According to the law of conservation of linear momentum,if the net external force acting on a system is zero $(F_{ext} = 0)$,then the total linear momentum $(P)$ of the system remains constant.
Additionally,the velocity of the center of mass $(v_{cm})$ remains constant because $P = M v_{cm}$.
Since the net external force is zero,the acceleration of the center of mass $(a_{cm})$ remains zero,as $F_{ext} = M a_{cm} = 0$.