Underline the correct alternative:
$(a)$ When a conservative force does positive work on a body,the potential energy of the body increases/decreases/remains unaltered.
$(b)$ Work done by a body against friction always results in a loss of its kinetic/potential energy.
$(c)$ The rate of change of total momentum of a many-particle system is proportional to the external force/sum of the internal forces on the system.
$(d)$ In an inelastic collision of two bodies,the quantities which do not change after the collision are the total kinetic energy/total linear momentum/total energy of the system of two bodies.

(A) Decreases
$(b)$ Kinetic energy
$(c)$ External force
$(d)$ Total linear momentum
$(a)$ $A$ conservative force does positive work on a body when it displaces the body in the direction of the force. As a result,the body moves toward the center of force,decreasing the separation and thus decreasing the potential energy.
$(b)$ Work done against friction reduces the velocity of a body,which results in a loss of kinetic energy.
$(c)$ Internal forces,according to Newton's third law,cancel each other out and cannot change the total momentum of a system. Therefore,the rate of change of total momentum is proportional to the external force.
$(d)$ In any collision (elastic or inelastic),the total linear momentum of the system remains conserved,provided no external force acts on the system.