If you were testing how well different automatic car washes cleaned cars, the control group would be:

I believe the answer is: The control group would be cars that were washed by hand

The control group exist in order to measure the result that created by subjects that recieve specal treatment (the special treatment in this case would be the automated car wash).

In the example above, the result from automatic car wash would be controlled by comparing it to the result from handwashed cars.