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

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Let the ball take t seconds to fall up to the bottom of the window.

It takes $(t- 0.1) $ seconds to fall up the top of window.

since the height (or length ) of window is 1 m , we have

$1= \large\frac{1}{2}$$gt^2-\large\frac{1}{2} $$g (t-0.1)^2 $

Solving, $t= 1.05 s$

The ball takes another 1 s to come upto the bottom of the building .

Therefore, the total time for t taken by the ball of fall from the roof of the building upto the bottom is $(1.05+ 1.00) =2.05$ .

Hence the height h of the building is given by

$h= \large\frac{1}{2} $$gt^2 =\large\frac{1}{2} $$ \times 10 \times 1.05^2 $

$\quad= 21.0 m$

Hence B is the correct answer.

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