The rate of flow of liquid in a tube of radius $r,$  length $ l,$  whose ends are maintained at a pressure difference $P$  is $V = \frac{{\pi QP\,{r^4}}}{{\eta l}}$ where $\eta $ is coefficient of the viscosity and $Q$ is
  • A$8 $
  • B$\frac{1}{8}$
  • C$16$ 
  • D$\frac{1}{{16}}$
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