Analogy type questions:
$(1)$ Reduction of nitrogen to ammonia : Biological nitrogen fixation : Nitrogenase : ........
$(2)$ Free-living bacteria : Rhodospirillum : Free-living blue-green algae : ........

• A
  $1$: Nitrogen reduction,$2$: Anabaena
• B
  $1$: Nitrogen oxidation,$2$: Nostoc
• C
  $1$: Nitrogen fixation,$2$: Rhizobium
• D
  $1$: Denitrification,$2$: Azotobacter