question_answer

A bird is sitting on the top of a vertical pole 20 m high and its elevation from a point O on the ground is \[45{}^\circ \]. It flies off horizontally straight away from the point O. After one second, the elevation of the bird from O is reduced to \[30{}^\circ \]. Then the speed (in m/sec) of the bird is

A)  \[20\sqrt{2}\]               

B)  \[40\,\,\left( \sqrt{2}-1 \right)\]

C)  \[40\,\,\left( \sqrt{3}-\sqrt{2} \right)\]       

D)  \[20\,\,\left( \sqrt{3}-1 \right)\]

E)  None of these