A `6 kg` mass travelling at `2.5 ms^(-1)` collides head on with a stationary `4 kg` mass. After the collision the `6kg` mass travels in its original direction with a speed of `1 ms^(-1)`. The final velocity of `4 kg` mass is
A. `1 ms^(-1)`
B. `2.25 ms^(-1)`
C. `2 ms^(-1)`
D. `0 ms^(-1)`