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