Answer:
The control group would be using no dish soap, just washing with water.
Explanation:
In order to test the effect of different soaps, it's necessary to first determine a few samples of soap, and using it on dishes that were dirtied with the same substances, using the same amount of the same temperature water. There should be a control group of dishes that would not be washed with soap, which will help determine if the treatment (in this case the soap applied) is more effective than no treatment.
