Consider elastic collision of a particle of mass m moving with a velocity u with another particle of the same mass at rest. After the collision the projectile and the struck particle move in directions making angles ${\mathrm{\theta }}_1$ and ${\mathrm{\theta }}_2$ respectively with the initial direction of motion. The sum of the angles. ${\mathrm{\theta }}_1+{\mathrm{\theta }}_2,$ is

• A

  45°

• B

  90°

• C

  135°

• D

  180°