A juggler is maintaining four balls in motion, making each in turn to rise to height of 20 m.Where will other three balls be at the instant, When one is just leaving his hand? $g= 10ms^{-2}$
$\begin{array}{1 1}(A)\;30m\\(B)\;20m \\(C)\;9 m \\(D)\;6m \end{array}$