The viscous force $F$ on a sphere of radius $a$ moving in a medium with velocity $v$ is given by F=6 $\pi \eta$ a v . The dimensions of $\eta$ are

• A

  $\left[{\mathrm{ML}}^{-3}\right]$

• B

  $\left[{\mathrm{MLT}}^{-2}\right]$

• C

  $\left[{\mathrm{MT}}^{-1}\right]$

• D

  $\left[{\mathrm{ML}}^{-1}{ \mathrm{T}}^{-1}\right]$