A block of mass `1 kg` moving with a speed of `4 ms^(-1)`, collides with another block of mass `2 kg` which is at rest. If the lighter block comes to rest after collision, then the speed of the heavier body is
A. `2 ms^(-1)`
B. `1 ms^(-1)`
C. `1.5 ms^(-1)`
D. `0.5 ms^(-1)`