Answer: Carbon dioxide.
- A Oxygen
- B Carbon dioxide
- C Nitrogen
- D Methane
Correct answer: B. Carbon dioxide
Explanation: Saccharomyces cerevisiae ferments the dough and releases CO<sub>2</sub>, which puffs up and leavens the bread.
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