Katherine wants to investigate if the time of day a plant is watered affects the growth of the plant. She waters the same type of plants with the same amount of water at the same time in the morning. The plants are sitting in the same location and get the same amount of sunlight. Which variable is missing from the experiment?
A. Independent variable: plants should be watered at different times of the day
B. Dependent variable: experiment should use different types of plants
C. Independent variable: the amount of sunlight should change daily
D. Controlled variable: experiment should use the same amount of water