To determine the effect of color of light on plant growth, 4 groups of bean plants were grown under the light conditions below.

Group 1—under red light Group 2—under green light

Group 3—under violet light Group 4—in normal, white light



Which of the variables below should be held constant?



Options:

The color of light each group of plants receives


All of the above


How much each plant grows


The amount of light each group of plants receives