Photosynthesis reduces CO2 levels in light Steps:

• Both tubes begin with high CO2, showing orange color.
• Tube 1 (light): Pondweed photosynthesizes, absorbing CO2 and lowering its concentration.
• Tube 2 (dark, black paper): No photosynthesis occurs; respiration releases CO2, keeping concentration high.
• After 3 hours, tube 1 turns purple (low CO2), tube 2 stays orange (high CO2).

Why C is correct:

• Photosynthesis consumes CO2 in light per the equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, lowering CO2 to produce purple; dark conditions limit this, maintaining orange via respiration.

Why the others are wrong:

• A: Ignores CO2 reduction in tube 1 from photosynthesis.
• B: Introduces undefined yellow for tube 2; dark maintains orange, not yellow.
• D: Reverses effects—light yields purple, not yellow; dark yields orange, not purple.

Final answer: C