Answer: c. the shower not sprayed with any mildew-preventing spray.
Step-by-step explanation:
The options in the question:
A. the shower sprayed with many test sprays
B. the shower not exposed to any mildew at all
C. the shower not sprayed with any mildew-preventing spray
D. the shower sprayed with a test spray that is already known to work.
The control group is the group in a study that isn't treated by the researcher but rather utilized as benchmark in order to know how the other groups that are tested are doing.
For the experiment in the above question, the control group will be showers that the mildew they're receiving contains water. It is the control group as it'll be compared with the group that's sprayed.