A person drops a stone from a building of height 20 m. At the same instant the front end of a truck passes below the building moving with constant acceleration of `1 m//s^(2)` and velocity of `2 m//s` at that instant. Length of the truck if the stone just misses to hit its rear part is :-
A. `6 m`
B. `4 m`
C. `5 m`
D. `2 m`
