Context: The passage below describes an experiment.

Nora wanted to find ways to save water. She learned that people can reuse greywater, which is water that has been used in sinks, tubs, and washing machines. She wondered if her plants would be less healthy if she watered them with greywater instead of tap water.
Nora placed eight potted petunia plants on her windowsill. For three months, she watered four of the plants with greywater and the other four with tap water. At the end of three months, she counted the number of dead leaves on each plant.
Figure: watering petunia plants.
In this experiment, which were part of an experimental group?
A. the plants watered with tap water
B. the plants watered with greywater
Select the correct answer from the given choices and respond with the letter of the chosen option.

B