A ball is thrown vertically downwards from a height of $20 m$ with an initial velocity $v_0$. It collides with the ground, loses 50 percent of its energy in collision and rebounds to the same height. The initial velocity $v_0$ is : (Take g = $10 ms^{–2}$)
( A ) $14 \; ms^{-1}$
( B ) $10 \; ms^{-1}$
( C ) $20 \; ms^{-1}$
( D ) $28\; ms^{-1}$