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 45o. It flies off horizontally straight away from the point O. After 1 s, the elevation of the bird from O is reduced to 30o. Then, the speed (in m/s) of the bird is

(a) 40 (√2 -1)

(b) 40 (√3 -√2)

(c) 20√2

(d) 20 (√3 -1)