Answer: Freely in the soil.
- A Symbiotically in root nodules
- B Freely in the soil
- C In partnership with legumes
- D Inside leaf cells
Correct answer: B. Freely in the soil
Explanation: Azotobacter is a free-living soil bacterium that fixes atmospheric nitrogen independently without forming symbiotic associations.
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