A ball is dropped from the roof of a building . An observer notes that the ball takes $0.1$ s to cross over a window 1 m in height . After crossing the window , the ball takes another 1.00 s to come to the bottom of the building . What is height of the building and how high is the window. Take $g=10 ms^{-2}$

Question

$\begin{array}{1 1}(A)\;30m\\(B)\;21\;m \\(C)\;9 m \\(D)\;67 m \end{array}$