Question 3
Why is an aquatic plant such as hydrilla used in this experiment?
The right answer is the one marked above.
Because a plant that already lives under water can be left there unharmed while the gas is collected. Working under water is what lets the bubbles be caught at all.
Vallisneria and hydrilla are the same plants used in activity 2.1 to watch chloroplasts move — convenient organisms, chosen for the experiment rather than for themselves.