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A scientist wants to see if a certain amount of water affects plant growth. He plants three seeds in three separate pots, with the same amount of soil, sunlight, and all at the same temperature. Pot 1 will receive no water, pot 2 will receive 1 cup of water a week, and pot 3 will receive 2 cups of water a week. After one month he records his observations.
What are three things the scientist must keep constant (not change) in this experiment?