# A particle is projected vertically up and it reaches a 15 cm high mark at certain instant. After 4 seconds from this instant the particle crosses this mark again in its downward journey. What is the initial velocity of projection of this particle? $(g= 10 ms^{-2})$
$\begin{array}{1 1}(A)\;10 \sqrt 7 \\(B)\;6 \sqrt 2 \\(C)\;5 \sqrt 5 \\(D)\;6.7 \sqrt 2 \end{array}$