A boy projects a stone vertically perpendicular to the trolley car with a speed v. If the trolley car moves with a constant velocity u, the time of flight of the stone is:

• A

  $\frac{\mathrm{u}+\mathrm{v}}{\mathrm{g}}$

• B

  $\frac{2\mathrm{v}}{\mathrm{g}}$

• C

  $\frac{2\mathrm{u}}{\mathrm{g}}$

• D

  none of these