Context: The passage below describes an experiment.

Dr. Christensen was the land manager for a prairie. The prairie had become overrun by unwanted grasses that were crowding out other plants. Dr. Christensen thought she could use fire to remove the unwanted grasses.
Dr. Christensen 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. Christensen 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 a control group?
A. the plots with no fires
B. the plots with fires
From the given choices, pick the correct answer and respond by indicating the letter of the correct option.

A