Julie is testing to see if increasing the Sunburn Protection Factor (SPF) of her sunscreen protects against sunburn better than the SPF 15 she always uses. Julie gets 4 identical panels of clear glass, UV sensitive paper (to indicate harmful sunlight), and 4 sunscreens of the same brand with different SPF. Julie smears equal amounts of SPF 15, SPF 30, SPF 45, and SPF 50 sunscreen evenly on the panels of glass. The glass panels are placed over UV sensitive paper in the sun at the same location for 4 hours.

What would be the best control group for Julie’s experiment? (SC.912.N.1.1)
A.a glass panel with the SPF 15 sunscreen
B.a glass panel with the SPF 30 sunscreen
C.a glass panel with the SPF 45 sunscreen
D.a glass panel with the SPF 50 sunscreen