Happy Holidays! :)

Looking at the information given in the description, the experiment is testing for the effects of the different salts on the growth of the plants.
This means that the type of salt is the independent variable, or the variable being manipulated and tested.
To ensure fair results, some parts of the experiment would need to be kept the same, which is the control. The only answer choice that makes sense in this instance is A. water.
I would think the last one about the ozone layer
This is called the pedigree chart.
C) Thermal energy. Your body turns the mechanical energy into thermal energy.
A covalent bond is your answer