In a one-dimensional collision between two particles, their relative velocity is `vecv_(1)` before the collision and `vecv_(2)` after the collision

Options

A. `vec(v_(1))=vec(v_(2))` if the collision is elastic
B. `vec(v_(1))=-vec(v_(2))` if the collision is elastic
C. `|vec(v_(1))|=|vec(v_(2))|` in all cases
D. `vec(v_(1))=-kvec(v_(2))` in all cases, where `kgt=1`