Context: The passage below describes an experiment.

Danny noticed that he could make a dome of water on a penny if he added drops of water slowly enough. He wondered if adding soap to the water would allow him to fit more or less water on the penny.
Danny put equal-sized drops of pure water, one at a time, onto a penny. He recorded the number of drops he could add before the water spilled over the edge of the penny. Then, he rinsed and dried the penny, and repeated the test using water mixed with hand soap. Danny repeated these trials on seven additional pennies.
Figure: a dome of water on the surface of a penny.
In this experiment, which were part of a control group?
A. the pennies with soapy water
B. the pennies with pure water
From the given choices, choose the correct answer and respond with the letter of that choice.

B