Take three test-tubes. Fill each of them 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 ?

Test-tube A will have the highest concentration of CO2 because snail will take in oxygen and gives out CO2. In test-tubes B and C, the CO2 will be utilized by the water plant for synthesizing food and hence there will be less concentration of CO2 in these.