Context: The passage below describes an experiment.

On winter mornings, Lucia had to scrape the ice off of the windshield and side mirrors of her car. Her friend told her that she should cover her side mirrors with plastic bags overnight to stop ice from forming.
One winter night, Lucia secured a plastic bag over one of the side mirrors on her car. She left the other side mirror uncovered. In the morning, she checked the percentage of each mirror that was covered by ice. Lucia repeated this test every night for one week, alternating which mirror she covered each night.
Figure: a side mirror covered in ice.
In this experiment, which were part of a control group?
A. the uncovered side mirrors
B. the covered side mirrors
From the given choices, choose the correct answer and respond with the letter of that choice.
Answer: A