Take three test-tubes. Fill each of each with water. Label them A, B and C. Keep a snail in test-tube A, a water plant in test-tube B and in C, keep snail and plant both. Which test-tube would have the highest concentration of CO₂?

In the described setup:
– Test-tube A contains a snail, which produces carbon dioxide (CO₂) through respiration.
– Test-tube B has a water plant that utilizes CO₂ during photosynthesis to produce oxygen.
– Test-tube C contains both a snail and a water plant.

The test-tube with the highest concentration of CO₂ would likely be Test-tube A, where the snail is present.

Here’s the rationale:
– Test-tube A (Snail): The snail undergoes respiration, releasing carbon dioxide as a byproduct. In the absence of a photosynthesizing organism (like the water plant), the CO₂ released from the snail’s respiration accumulates in the water, leading to a higher concentration of carbon dioxide.

– Test-tube B (Water Plant): The water plant consumes CO₂ during photosynthesis and produces oxygen. While it may initially release some CO₂ during the absence of light (dark respiration), overall, the net production of oxygen might exceed the small amount of CO₂ produced.

– Test-tube C (Snail and Plant): Here, the water plant absorbs some of the carbon dioxide produced by the snail during respiration and uses it for photosynthesis. The net concentration of CO₂ might be lower compared to the test-tube with only the snail.

Therefore, the test-tube with the highest concentration of CO₂ is likely Test-tube A, where only the snail is present, as it continually releases CO₂ through its respiration process without any counterbalancing effect of photosynthesis.