In a conductivity cell,the electrodes are separated by a distance of $2 \ cm$ and have a cross-sectional area of $4 \ cm^2$. If the conductivity of pure water is $8 \times 10^{-7} \ S \ cm^{-1}$,then the resistance of the water is:

• A
  $8.13 \times 10^{-7} \ \Omega$
• B
  $6.78 \times 10^{-5} \ \Omega$
• C
  $6.25 \times 10^{5} \ \Omega$
• D
  $7.18 \times 10^{7} \ \Omega$