Marie notices that the current cleaner she is using on her shower is not cutting through the hard water build-up that has accumulated. She decides to purchase 3 other cleaners to see if one of them will do a better job of cutting through the stains. Marie places masking tape in the dry shower to create 5 sections for her experiment. She sprays one cleaner per section and leaves the remaining section untouched to compare her results. After 30 minutes, and without scrubbing any of the cleaner in, she records her observations. In this example, what is the dependent variable?