A long straight wire of resistance R, radius 'a' and length 'L' carries a constant current "I". The poynting vector for the wire is

• A

  $\frac{\mathrm{IR}}{2\mathrm{\pi al}}$

• B

  $\frac{{\mathrm{IR}}^2}{\mathrm{al}}$

• C

  $\frac{{\mathrm{I}}^2\mathrm{R}}{\mathrm{al}}$

• D

  $\frac{{\mathrm{I}}^2\mathrm{R}}{2\mathrm{\pi al}}$