Testing CO2 necessity by absorption in photosynthesis

Steps:

• Determine experiment goal: remove CO2 and check if photosynthesis (oxygen production or starch formation) stops in light.
• Identify CO2 removal method: sodium hydroxide absorbs CO2 to create deficient conditions.
• Confirm setup includes light for photosynthesis, airtight seal to trap gases, plant, and water.
• Eliminate options lacking CO2 absorption or proper conditions.

Why A is correct:

• Sodium hydroxide absorbs CO2, testing its role in the photosynthesis equation (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂) by preventing reaction without it.

Why the others are wrong:

• B: Darkness inhibits photosynthesis entirely, failing to isolate CO2's effect.
• C: Hydrogencarbonate indicator only detects CO2 levels, not removes it for testing necessity.
• D: Lacks any CO2 manipulation, so cannot demonstrate its requirement.

Final answer: A