If between wavelength $\lambda$ and $\lambda + d\lambda$,$e_\lambda$ and $a_\lambda$ are the emissive and absorptive powers of a body,respectively,and $E_\lambda$ is the emissive power of a perfectly black body,then according to Kirchhoff's law,which of the following is true?

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