The electrical resistance $R$ of a conductor of length $l$ and area of cross-section $a$ is given by $R = \frac{\rho l}{a}$,where $\rho$ is the electrical resistivity. What is the dimensional formula for electrical conductivity $\sigma$,which is the reciprocal of resistivity?

• A
  $[M^{-1} L^{-3} T^3 A^2]$
• B
  $[M L^{-3} T^{-3} A^2]$
• C
  $[M L^3 T^{-3} A^{-2}]$
• D
  $[M^{-2} L^3 T^2 A^{-1}]$