Hydrogencarbonate indicator detects net CO2 balance from photosynthesis and respiration

Steps:

• Hydrogencarbonate indicator turns purple with low CO2 (net photosynthesis), stays orange at neutral CO2 (balanced rates), and yellow with high CO2 (net respiration).
• Option A: Plant in darkness respires only, increasing CO2 to yellow.
• Option B: Plant in light stays orange, showing no net CO2 change.
• Option C: Plant in light turns purple, indicating CO2 uptake exceeds release.
• Option D: Snail in light respires only, increasing CO2 to yellow.

Why B is correct:

• Orange color indicates neutral pH from equal CO2 production (respiration) and consumption (photosynthesis), per indicator's CO2 sensitivity.

Why the others are wrong:

• A: Darkness prevents photosynthesis, so respiration raises CO2.
• C: Light enables photosynthesis to outpace respiration, lowering CO2.
• D: Snail lacks photosynthesis, so respiration alone raises CO2.

Final answer: B