If $e_\lambda$ and $a_\lambda$ are the emissive power and absorptive power of a body respectively,and $E_\lambda$ is the emissive power of a perfectly black body,what is the relation according to Kirchhoff's law?

• A
  $a_\lambda = \frac{E_\lambda}{e_\lambda}$
• B
  $\frac{a_\lambda}{e_\lambda} = E_\lambda$
• C
  $\frac{e_\lambda}{a_\lambda} = E_\lambda$
• D
  $e_\lambda = \frac{E_\lambda}{a_\lambda}$