Caroline noticed that her shower curtain is covered in green mildew , her roommate tells her that vinegar will get rid of it , caroline decides to test this by spraying one half of the shower curtain with 100 ml of vinegar and the other half with 100 ml of plain water , after letting both sides soak for 3 days there was no change in the appearance of the mildew on both either side of the shower curtain what is the independent variable ?