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

• A

  $20\sqrt{2}$
• Correct answer
  B

  $20\left(\sqrt{3}-1\right)$
• C

  $40\left(\sqrt{2}-1\right)$
• D

  $40\left(\sqrt{3}-\sqrt{2}\right)$

Hence, the correct answer is option: 'B'.