Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: A student hypothesizes that the amount of water a plant receives will affect how long it grows before reaching maturity. To test this hypothesis, the student will need
several identical plant seeds.
Control group, variable, dependent variable, independent variable.