Answer:
d the net external force acting on the two-body system is zero
Explanation:
The net external force acting on the two-body system is zero .
During a collision , two internal forces appear on two bodies in opposite directions separately . As they act on each object separately , they do not cancel each other . But the net force on two body system is zero . Total momentum is always conserved during collision because no external force is involved . Collision is not always elastic .