A ladder, 5 meter long,standing on a horizontal floor,leans against a vertical wall.If the top of the ladder slides downwards at the rate of 10cm/sec,then the rate at which the angle between the floor and the ladder is decreasing when lower end of ladder is 2 meters from the wall measure in "radian/seconds" is?

Question

\[(A)\;\frac{1}{10}radian/sec\quad(B)\;\frac{1}{20}radian/sec\quad(C)\;20\;radian/sec\]\[(D)\;10\;radian/sec\]