A stone is projected with a velocity $10\sqrt{2}{\mathrm{ms}}^{-1}$ at an angle of 45° to the horizontal ground. The average velocity of stone during its time of flight is 

• A

  $10 {\mathrm{ms}}^{-1}$

• B

  $10\sqrt{5}{\mathrm{ms}}^{-1}$

• C

  $5\sqrt{5}{\mathrm{ms}}^{-1}$

• D

  $20 {\mathrm{ms}}^{-1}$