Biology

A student is testing the effect of different water temperatures on the germination rate of bean seeds. She uses three groups: one group is watered with cold water (5°C), another with room-temperature water (20°C), and the third with warm water (35°C). She ensures that all the seeds receive the same amount of water, are planted in the same type of soil, and are placed in the same environment.

Which of the following is a constant in this experiment?