A box of mass 9kg is placed on a rough inclined plane of inclination $\mathrm{\theta }$. Its downward motion can be prevented by applying an upward pull of 20N and it can be made to slide upwards by applying a force 40N. The coefficient of friction between the box and the inclined plane is 

• A

  $\frac{1}{3}\tan \mathrm{\theta }$

• B

  $3\tan \mathrm{\theta }$

• C

  $\frac{1}{2}\tan \mathrm{\theta }$

• D

  $2\tan \mathrm{\theta }$