Context: The passage below describes an experiment.

Layla 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. Layla wondered if steel would rust faster if she added vinegar to the salt water.
Layla 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, Layla 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 an experimental group?
A. the steel squares soaked in salt water
B. the steel squares soaked in salt water and vinegar
Identify the correct choice from the options below and respond with the letter of the correct option.

B