Turpentine oil is flowing through a tube of length $l$ and radius $r$. The pressure difference between the two ends of the tube is $P$. The viscosity of oil is given by $\eta = \frac{P(r^2 - x^2)}{4vl}$,where $v$ is the velocity of oil at a distance $x$ from the axis of the tube. The dimensions of $\eta$ are

• A
  $[MLT^{-1}]$
• B
  $[M^0L^0T^0]$
• C
  $[ML^{-1}T^{-1}]$
• D
  $[ML^2T^{-2}]$