A block of mass m is stationary on a horizontal surface. It is connected with a string which has no tension. The coefficient of friction between the block and surface is m. Then, the frictional force between the block and surface is:

• A

  zero

• B

  $\mathrm{\mu } \mathrm{mg}$

• C

  $\frac{\mathrm{mg}}{\mathrm{\mu }}$

• D

  none of these