Two railway trucks with masses $m$ and $3m$ are travelling in opposite directions towards each other with speeds $2v$ and $v$ respectively. They collide and then remain stuck together. What is the speed of the trucks immediately after the collision?
- A$\frac{v}{4}$
- B$\frac{v}{2}$
- C$v$
- D$\frac{5v}{4}$