Conductivity (unit $S$) is directly proportional to the area of the vessel $(A)$ and the concentration of the solution $(C)$ in it,and is inversely proportional to the length of the vessel $(l)$. The unit of the constant of proportionality is:

• A
  $S \, m \, mol^{-1}$
• B
  $S \, m^2 \, mol^{-1}$
• C
  $S^{-2} \, m^2 \, mol$
• D
  $S^2 \, m^2 \, mol^{-2}$