Mildew is a fungus that grows in wet areas and can slowly grow from microscopic to large stains in showers and sinks. Researchers are studying different sprays that could be used to slow down the growth of mildew. They set up different showers that are exactly the same and expose them each to water with the same microscopic amount of mildew. They then spray some showers with mildew-preventing sprays and observe how fast mildew grows over time. Which shower would most likely be the control group in this study?

the shower sprayed with many test sprays
the shower not exposed to any mildew at all
the shower not sprayed with any mildew-preventing spray
the shower sprayed with a test spray that is already known to work