[单选题]
Two objects collide and bounce off each other. Linearbgkj;xfuhcg;j *h z9:(/b(i y:ar bc/ momentuy/w, qy2949lj nxerd gm
A. is definitely conserved.
B. is definitely not conserved.
C. is conserved only if the collision is elastic.
D. is conserved only if the environment is frictionless.