The area of cross-section of a potentiometer wire is $6 \times 10^{-7} \ m^2$. The potential difference per unit length of the potentiometer wire when it is connected to a cell of negligible internal resistance and a resistor in series is $0.15 \ Vm^{-1}$. If the current through the potentiometer wire is $0.3 \ A$,then the resistivity of the material of the potentiometer wire is:

• A
  $4 \times 10^{-6} \ \Omega \ m$
• B
  $3 \times 10^{-7} \ \Omega \ m$
• C
  $3 \times 10^{-6} \ \Omega \ m$
• D
  $4 \times 10^{-7} \ \Omega \ m$