Test-tube A will have the highest concentration of CO2 because :
(1) Test-tube A will have a snail that expels out CO2 into the tube. Living organisms use oxygen and release CO2 during respiration.
(2) Since a plant is present in both test tubes B and C, the plant will inhale CO2 to decrease CO2 concentration.
(3) Thus, there will be less concentration of CO2 in B and C test tubes.