A bullet having a speed of `100 m//see` crashes through a plank of wood. After passing through a plank , its speed is `80 m//s`. Another bullet of the same mass and size , but traveling at `80 m//s` is fired at the plank . The speed of the second bullet after traveling through the plank is (Assume that resistance of the plank is independent of the speed of the bullet) :
A. `10 sqrt(7) ms^(-1)`
B. `20 sqrt(7) ms^(-1)`
C. `30 sqrt(7) ms^(-1)`
D. `20 sqrt(7) ms^(-1)`