The rate of flow of liquid in a tube of radius r, length l, whose ends are maintained at a pressure difference P is $\mathrm{V}=\frac{\mathrm{\pi QP}{\mathrm{r}}^4}{\mathrm{\eta l}}$  where $\mathrm{\eta }$  is coefficient of the viscosity and Q is 

• A

  8

• B

  $\frac{1}{8}$

• C

  16

• D

  $\frac{1}{16}$