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/s) of the bird is

Question

( A ) $20\sqrt{2}$
( B ) $20 (\sqrt{3}-\sqrt{2})$
( C ) $20 (\sqrt{3}-1)$
( D ) $40 (\sqrt{2}-1)$