Balls X and Y move towards one another along the same straight line and then collide. The collision is perfectly elastic. Their initial speeds are $u_x$ and $u_y$ respectively. After the collision, they separate with speeds $v_x$ and $v_y$ respectively. Their directions are shown. Which equation is right?
- A$u_x + u_y = v_x + v_y$
- B$u_x + u_y = v_x - v_y$
- C$u_x - u_y = v_x + v_y$
- D$u_x - u_y = v_x - v_y$