Two railway trucks, with masses $m$ and $3m$, travel in opposite directions towards one another at speeds $2v$ and $v$ respectively. They collide and remain stuck together. What is the speed of the trucks after the collision?
- A$\frac{v}{4}$
- B$\frac{v}{2}$
- C$v$
- D$\frac{5v}{4}$