Question 2
Why is leaf B covered with colourless polythene rather than left uncovered?
The right answer is the one marked above.
So that the two leaves differ in one thing only — whether light reaches them. Leaf B is the control: it has polythene on it too, so the polythene itself cannot explain the difference.
If only leaf A were covered, someone could argue the plastic blocked the air or trapped heat. Leaf B answers every such objection at once.