Read the scenario.
You want to answer the following question:
Does drinking cause liver damage?
You set up an experiment by selecting a group of people and dividing them into two groups. You test each group for liver damage and find that there is none in either group. Then, you allow Group A to live as usual, but you have Group B drink 24 ounces of alcohol a day for five years. After five years, you test both groups and find that there is more liver damage in Group B.
What was the independent variable in your experiment?
the passage of five years
the consumption of alcohol
Since we are looking for the independent variable I am pretty sure the answer is C