The force $(F)$ required to maintain the flow of layers of a liquid is equal to ($A = \text{area of contact of layers}$, $dz = \text{distance between the layers}$, $du = \text{change in velocity}$, $\eta = \text{coefficient of viscosity}$)

• A
  $\eta \frac{du}{dz} \cdot \frac{1}{A}$
• B
  $\eta \frac{dz}{du} \cdot A$
• C
  $\eta A \frac{du}{dz}$
• D
  $\eta \frac{dz}{A} \cdot \frac{1}{du}$