Does Atmospheric Pressure Have an Impact on Plant Heights? to Study

Does atmospheric pressure have an impact on plant heights? To study this question, 20 Spartina alterniflora plants were planted at sea level (1 atmosphere) , while another 20 Spartina alterniflora plants were grown in a special chamber aboard the International Space Station (ISS) (0 atmospheres) . The height of the plants after 3 months was recorded. The goal was to see whether the plant heights (in centimeters) were affected by the atmospheric pressure exerted on the plant.
What is the response variable in this study?

A) Location in which the plants were grown
B) Number of months the plants were observed
C) Heights of the plants
D) Amount of atmospheric pressure exerted on the plants