Ascientific study looked at the effect of tanning beds on dna damage. the scientists took skin cells and exposed them to uv radiation (the type used in indoor tanning beds) for different lengths of time: some for 1 minute, some for 5 minutes, some for 15 minutes, and some for 30 minutes. they then looked for signs of dna damage and compared the results to cells that had never been exposed to uv light. what is the independent variable in this experiment