The bowl has more volume, the bearing has more volume. The mass is bigger for the bearing because it is heavier than the bowl. It is made of metal and the weight of it is greater than the bowl.(the body shape).
I think it’s microbiology, since it would help the student understand the process of composting the best. Botany is plants- not for composting. Geophysics- he’s not researching tectonic plates. Oceanography- he isn’t interested in the marina trench
Sorry this will probably be pretty long.
So think of the "control" as being something you yourself add to increase or decrease the effects in an experiment.
I'll give you an example so it is not as confusing.
Say you have decided to make an experiment on plants. Which plant can grow the fastest on which type of liquid? What is being added to this experiment? The liquid! Or all of the liquids you used. Like if you used Coke, Lime Gatorade, Orange Gatorade, and Water. Each drink will EFFECT each plant differently.
Hope I was of any hope?