Context: The passage below describes an experiment.

Dr. Moran was the land manager for a prairie. The prairie had become overrun by unwanted grasses that were crowding out other plants. Dr. Moran thought she could use fire to remove the unwanted grasses.
Dr. Moran marked off six plots within a large area of the prairie. In three of the plots, she set a carefully controlled fire to burn all of the plants once each year for three years. In the remaining three plots, she did not set any fires. A year after the last fire, Dr. Moran estimated the percentage of area covered by unwanted grasses in each of the six plots.
Figure: a fire burning a prairie.
In this experiment, which were part of an experimental group?
A. the plots with fires
B. the plots with no fires
Answer with the option's letter from the given choices directly.

A