71st with reason

​Q71. The coefficient of viscosity ($\mathrm{\eta }$) of a liquid by the method of flow through a capillary tube is given by the formula $\mathrm{\eta }$ = $\frac{\mathrm{\pi } \mathrm{R} \mathrm{P}}{8 \mathrm{l} \mathrm{Q}}$
  where R = radius of the capillary tube, 
             l = length of the tube and 
             P = pressure difference between its ends, and 
             Q = volume of liquid flowing per second.
     Which quantity must be measured most accurately?

        (A) R                          (B) l                     (C) P                     (D) Q