Two spheres travel towards one another along a single straight line. Before the collision, their speeds are $u_1$ and $u_2$. After the collision, the spheres separate with speeds $v_1$ and $v_2$ in the directions shown below. Which equation has to be true if the collision is perfectly elastic?
- A$u_1 - u_2 = v_2 + v_1$
- B$u_1 - u_2 = v_2 - v_1$
- C$u_1 + u_2 = v_2 + v_1$
- D$u_1 + u_2 = v_2 - v_1$