A student was interested in comparing sunblock lotions, which contain chemicals that protect the skin from sunburn. The student purchased three
different brands of lotion at three different prices and performed an
experiment to see which one best protected the skin. The lotions cost $3,
$5, and $7 for an 8-ounce bottle. The sun protection factor (SPF) was 30 for
all three lotions. The student applied equal amounts of each lotion next to
each other on one of her arms. The three areas covered by the lotions were
the same size. The student observed the three areas for color change after
two hours of Sun exposure. Identify one condition that were held constant
in this experiment. *