Context: The passage below describes an experiment.

Lester worked in a restaurant that had a vegetable garden. Part of his job was to care for the tomato plants. He had heard that pruning, or trimming, tomato plants can help tomatoes grow. He wondered if pruning would affect how well his tomato plants grew.
Lester chose 24 tomato plants that were similar in size. He pruned an equal number of branches from each of the first 12 plants. He left the other 12 plants unpruned. Then, each week, Lester weighed the tomatoes that he picked from each group of plants.
Figure: a tomato plant.
In this experiment, which were part of an experimental group?
A. the unpruned tomato plants
B. the pruned tomato plants
Pick the correct answer from the listed options and provide the letter of the selected option.
Answer: B