The answer is #4. The first and second one involve acid, which is chemical weathering. The third one involves oxidation, which is a chemical process. The only mechanical process is option four.
Hypothesis, is the answer
The independent variable is what will change. The dependent variable is what will change because of the independent variable. The controlled variable is what will stay the same.
Ex, for the first one.
IV: the amount of sunlight
DV: plant growth
CV: pea plant
For #2.
IV: the soda
DV: the amount of sugar
CV: soda
And so on
The substance is called DNA.
