Ans a) is he wants to show that a plant takes up water through its roots. But b) isn’t a good question at all. The model answer is C, by the way.
First, kids are taught in experiments, we need to set up controls, and change only one variable. This means, in each experiment, there must be at least 2 setups. No single jar by itself shows conclusively, on its own, that plants take up water.
Second, arguably, we should compare Jar A with Jar B, since one has a plant and one doesn’t, and all else is the same.
Third, we can only compare Jar C with Jar B, since one has an oil layer and a plant, while one only has a plant. This doesn’t conclusively show anything except that more water was lost probably due to evaporation.
Finally, in order to properly use Jar C, we need a fourth setup (lets call Jar D) which is water, oil and no plant. Only then, can we conclusively say that water is taken up by the plant.
I’m frustrated because we try teaching the right things, but questions like these throw all of us off. Teachers are then supposed to try to help kids tackle these questions as best as they can when it’s not a good question at all. Sigh “