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Two particles $A$ and $B$, move with constant velocities ${\vec v_1}$ and ${\vec v_2}$. At the initial moment their position vectors are ${\vec r_1}$ and ${\vec r_2}$ respectively. The condition for their collision is

• A
  ${\vec r_1} - {\vec r_2} = {\vec v_1} - {\vec v_2}$
• ✓
  $\frac{{{{\vec r}_1} - {{\vec r}_2}}}{{\left| {{{\vec r}_1} - {{\vec r}_2}} \right|}} = \frac{{{{\vec v}_2} - {{\vec v}_1}}}{{\left| {{{\vec v}_2} - {{\vec v}_1}} \right|}}$
• C
  ${{\vec r}_1}.{{\vec v}_1} = {{\vec r}_2}.{{\vec v}_2}$
• D
  ${{\vec r}_1} \times {{\vec v}_1} = {{\vec r}_2} \times {{\vec v}_2}$