Context: The passage below describes an experiment.

Jessica was using steel to make rusted sculptures. After building each sculpture, she caused it to rust by placing it in salt water for eight hours. Jessica wondered if steel would rust faster if she added vinegar to the salt water.
Jessica cut ten squares of steel sheet metal. She put five of the squares into a tub filled with salt water. She put the other five squares into a tub filled with salt water mixed with vinegar. Once an hour for eight hours, Jessica counted how many steel squares in each group had rust on them.
Figure: a sculpture made from rusted steel.
In this experiment, which were part of a control group?
A. the steel squares soaked in salt water
B. the steel squares soaked in salt water and vinegar
From the given choices, select the correct answer and reply with the letter of the chosen option.
Answer: A