A platform of infinite mass is moving upward with velocity `5 m//s.` At time t = 0, a ball which is at height 100 m above the platform starts falling freely. The velocity of ball just after the collision will be (Assume elastic collision) `(g=10 m//s^(2)):`
A. `40 m//s`
B. `50 m//s`
C. `20m//s`
D. None
