A block has been placed on an inclined plane. The slope angle $\mathrm{\theta }$ of the plane is such that the block slides down the plane at a constant speed. The coefficient of kinetic friction is equal to:

• A

  sin $\mathrm{\theta }$

• B

  cos $\mathrm{\theta }$

• C

  g

• D

  tan $\mathrm{\theta }$