A car moves from X to Y with a uniform speed ${\mathrm{\upsilon }}_{\mathrm{u}}$ and returns to Y with a uniform speed ${\mathrm{\upsilon }}_{\mathrm{d}}.$ the average speed for this round trip is:

• A

  $\frac{2{\mathrm{\upsilon }}_{\mathrm{d}}{\mathrm{\upsilon }}_{\mathrm{u}}}{{\mathrm{\upsilon }}_{\mathrm{d}}+{\mathrm{\upsilon }}_{\mathrm{u}}}$

• B

  $\sqrt{{\mathrm{\upsilon }}_{\mathrm{u}}{\mathrm{\upsilon }}_{\mathrm{d}}}$

• C

  $\frac{{\mathrm{\upsilon }}_{\mathrm{u}}{\mathrm{\upsilon }}_{\mathrm{d}}}{{\mathrm{\upsilon }}_{\mathrm{d}}+{\mathrm{\upsilon }}_{\mathrm{u}}}$

• D

  $\frac{{\mathrm{\upsilon }}_{\mathrm{u}}+{\mathrm{\upsilon }}_{\mathrm{d}}}{2}$