the type of soil they use the amount of water they use the color of the pots they use the color of light bulb they use
I gave my students this multiple choice question: A group of students does an experiment to see if different colors of light affect the number of flowers a plant produces. Their teacher gives them seeds, pots, soil, water, and three light bulbs of different colors. The students decide to divide the materials equally. What should be different for each group of plants?

Only one answer is correct among these 4 choices:
- the type of soil they use
- the amount of water they use
- the color of the pots they use
- the color of light bulb they use

Could you tell me which one is correct?

the color of light bulb they use