A rocket rises vertically up from the surface of earth so that it is distance veries from the earth's surface as `l=bt^(2)` where 'b' is a constant. After `10 sec` the rocket has travelled `2 Km`. Determine it's speed at that moment.

Options

A. `100 m//s`
B. `200 m//s`
C. `400 m//s`
D. `600 m//s`

Answer

C

Explanation

distance `=l=bt^(2)=2000`
velocity `=(dl)/(dt) =2bt=2t.((l)/(t^(2)))=400m//s`