JEE Math Practice Question

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 files 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 :

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