$A$ highly viscous liquid of viscosity coefficient $\eta$ flows through a fixed horizontal cylindrical tube (fixed from the outer surface) of internal radius $r$,thickness $t$ $(t \ll r)$,and length $l$. The volume of liquid flowing per second is $Q$ and the pressure difference across the tube is $P$. If the modulus of rigidity of the material of the tube is $\beta$,the shear strain in the tube will be:

• A
  $\frac{8\eta Q}{\pi \beta r^2}$
• B
  $\frac{4\eta Q}{\pi \beta r^3}$
• C
  $\frac{\pi \beta r^3}{16\eta Q}$
• D
  $\frac{\beta r^2}{8\eta Q}$