Conceptual physics

Classical Mechanics Level 4

Two particles move with constant velocities $v_1$ and $v_2$ . Initially their radius vectors are $r_1$ and $r_2$ . For the two particles to collide, how much the four vectors be related?

(v1 - v2) / I v1- v2 I = (r1 - r2) / I r1 -r2 I (v1 - v2) / I v1+ v2 I = (r1 - r2) / I r1 +r2 I (v1 + v2) / I v1+ v2 I = (r1 + r2) / I r1 +r2 I (v1 + v2) / I v1- v2 I = (r1 + r2) / I r1 -r2 I