Need help with finding my control variable for Science Fair?

What is my control variable if my testable question is: Which SPF lotion is most effective, 8, 30, or 45+?|||Your control is the skin sample with no SPF lotion at all-- this is what no protection would look like (completely ineffective). Alternatively, your control can be a skin sample with no expose to sunlight (completely effective protection). It just depends on how you are going to test your question and display your results. I would pick the first one though-- a strongly burned control sample will show how well each protecting sample is working in comparison.