pls help.

Dr. AT is studying the effect playing music has on plant growth. To do this she sets up three greenhouses each growing pea plants. One of the greenhouses plays classical music to the plants all day, the second greenhouse plays pop music, while the third is silent. All other factors, such as water and sunlight are identical between the three greenhouses. After two weeks, Dr. AT measures each pea plant and records her data. What greenhouse is the control group in this experiment?

A. The silent greenhouse.
B. The pop music greenhouse.
C. The classical music greenhouse.
D. The height of the plants.