Answer: Anabaena.
- A Nostoc
- B Anabaena
- C Spirulina
- D Oscillatoria
Correct answer: B. Anabaena
Explanation: Anabaena forms a symbiotic association with the water fern Azolla and fixes atmospheric nitrogen, benefiting paddy crops.
Sewage passes through physical removal (primary), then aerobic biological treatment where microbes form flocs that consume organic matter (secondary); the settled activated sludge is then digested anaerobically to produce biogas - a deliberate switch from aerobic to anaerobic conditions between stages.
Concept context
Microorganisms in food, medicine, sewage treatment, biocontrol, and biofertilizers