age of seedlings temperature of water volume of water number of hours in sunlight
I gave my students this multiple choice question: A student has decided to investigate whether the number of flowers on a plant will increase if the water supply to the plant is increased. She has five pots of geranium plants to use in her experiment. What factor in the experiment should be varied for the five plants in order to answer the student's question?

Only one answer is correct among these 4 choices:
- age of seedlings
- temperature of water
- volume of water
- number of hours in sunlight

Could you tell me which one is correct?

volume of water