A disc with mass $M$ travels over a horizontal surface that is frictionless at a constant speed $u$. It then collides with a second disc of mass $4M$, which is initially at rest. The diagram gives a plan view of the two discs’ motion before and after the collision. Determine the initial velocity $u$ of the disc of mass $M$.
- A$1.1\,\text{m s}^{-1}$
- B$1.4\,\text{m s}^{-1}$
- C$3.5\,\text{m s}^{-1}$
- D$5.7\,\text{m s}^{-1}$