A body of mass $m$, travelling with speed $u$ on a smooth horizontal surface, undergoes a head-on collision with a body of mass $4m$ that is initially at rest. Following the collision, the mass-$m$ body moves back along the line of approach with $\frac{1}{4}$ of the kinetic energy it had before the impact. What is the speed of the mass-$4m$ body after the collision?
- A$\frac{u}{8}$
- B$\frac{3u}{16}$
- C$\frac{5u}{16}$
- D$\frac{3u}{8}$