Context: The passage below describes an experiment.

On winter mornings, Elizabeth 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, Elizabeth 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. Elizabeth 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 an experimental group?
A. the uncovered side mirrors
B. the covered side mirrors
Determine the correct option from the provided choices and reply with its corresponding letter.

B