A juggler is maintaining four balls in motion, making each in turn to rise to height of 20 m. With what velocity does he project them ?

Question

$\begin{array}{1 1}(A)\;30ms^{-1}\\(B)\;20ms^{-1} \\(C)\;9 ms^{-1} \\(D)\;67 ms^{-1} \end{array}$