Rates of Reactions

I am conducting a experiment on increasing the rate of a reaction by increasing the surface area. For this I am reacting hydrochloric acid and marble chips.

Variables:-
-Independent: the mass of marble chips
-Dependent: The volume of gas
-Controlled: Temp of HCL & Mass of marble chips

One aspect I am finding difficult to write about is:

1) to use scientific theory to explain why the rate of a reaction is affected by the variable.

2) to show detailed knowledge and understanding of experiments to measure the rate of reaction.

3)to discuss four variables that could be investigated.

Please help on these 3 aspects.
Thank You