A ball is projected vertically down with an initial velocity from a height of 20 m on to a horizontal floor. During the impact it losses 50% of the energy and rebounds to the same height, the initial velocity of its projection is :

Question

( A ) $10\;ms^{-1}$
( B ) $5\; ms^{-1}$
( C ) $20\;ms^{-1}$
( D ) $15\;ms^{-1}$