Biological nitrogen fixation is conversion of

• A

  ${\mathrm{N}}_2⟶{\mathrm{NH}}_3$

• B

  ${\mathrm{NH}}_3⟶{\mathrm{NO}}_2^{-}$

• C

  ${\mathrm{NO}}_2^{-}⟶{\mathrm{NO}}_3^{-}$

• D

  ${\mathrm{NO}}_3^{-}⟶{\mathrm{N}}_2/{\mathrm{NH}}_3$