A car of mass $400\; kg$ and travelling at $72\; km/h$ crashes into a truck of mass $4000\; kg$ and travelling at $9 \; km/h$, in the same direction. The car bounces back at a speed of $18\; km/h$. The speed of the truck after the impact is :

Question

( A ) $18 \;km/h$
( B ) $36 \;km/h$
( C ) $27 \;km/h$
( D ) $9 \;km/h$