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If between wavelength $\lambda $and $\lambda + d\lambda $, ${e_\lambda }$and ${a_\lambda }$ be the emissive and absorptive powers of a body and ${E_\lambda }$ be the emissive power of a perfectly black body, then according to Kirchoff's law, which is true

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