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 $\;45^{0}\;$. It flies off horizontally straight way from the point O . After one second ,the elevation of the bird from O is reduced to $\;30^{0}\;$. Then the speed in $(m/s)\;$ of the bird is :

Question

$(a)\;20 \sqrt{2}\qquad(b)\;20 (\sqrt{3}-1)\qquad(c)\;40(\sqrt{2}-1)\qquad(d)\;40(\sqrt{3}-\sqrt{2})$