For a sphere moving through a fluid, the frictional force $F$ is defined by $F = 6\pi a \eta v$, where $a$ represents the sphere’s radius, $\eta$ is a constant connected with the fluid, and $v$ is the sphere’s speed. What units does $\eta$ have?
- A$\text{kg m s}^{-1}$
- B$\text{kg m}^{-1}\text{s}^{-1}$
- C$\text{kg m s}^{-3}$
- D$\text{kg m}^{3}\text{s}^{-3}$