A boy walks to his school at a distance of 6 km with constant speed of 2.5 km/h and walks back with a constant speed of 4 km/h. His average speed for the round trip is

• A

  $5\mathrm{km}{\mathrm{h}}^{-1}$

• B

  $\frac{40}{13}\mathrm{km}{\mathrm{h}}^{-1}$

• C

  3 km/h

• D

  $\frac{1}{2}\mathrm{km}{\mathrm{h}}^{-1}$