$A$ block of mass $m$ lying on a rough horizontal plane is acted upon by a horizontal force $P$ and another force $Q$ inclined at an angle $\theta$ to the vertical. The minimum value of the coefficient of friction between the block and the surface for which the block will remain in equilibrium is:

• A
  $\frac{P \cos \theta+Q}{m g-Q \sin \theta}$
• B
  $\frac{P+Q \sin \theta}{m g+Q \cos \theta}$
• C
  $\frac{P+Q \cos \theta}{m g+Q \sin \theta}$
• D
  $\frac{P \sin \theta-Q}{m g-Q \cos \theta}$