Answer:
The temperature the plants are grown in
The amount of water the sunflowers get
The amount of light the sunflowers get
Explanation:
They want to know if increasing the acidity of the soil makes the plants flower sooner. So, they need to hold constant all the other variables in that area not the acidity of the soil or the day the plants begin to flower.
Therefore, the answers are
The temperature the plants are grown in
The amount of water the sunflowers get
The amount of light the sunflowers get
