A car of mass 1000 kg travelling at 8.0 m / s collides with a lorry of mass 3000 kg that is travelling at 2.0 m / s in the same direction. After colliding, the two vehicles stick together. What is their speed after the collision? A 2.0 m / s B 2.5 m / s C 3.5 m / s D 5.0 m / s