time difference in dropping of the stones is $t =\frac{2 h }{ g }=1\; s$
$h =\frac{1}{2} g t ^{2}=\frac{1}{2} g ( t -1)^{2}+25 \Rightarrow t =3 \;s$
thus, $h =\frac{1}{2} g t ^{2}=\frac{1}{2} \times 10 \times (3)^{2}= 4 5 \;m$
$OA, \,AB,\, BC,\, CD$