The control variable that can be used in the research experiment is to ensure that the two set of plants will receive the same amount of water and light.
According to the information, the researcher wants to seek whether singing to plants causes them to grow taller.
The explanatory variable is singing. Therefore, the researcher can control some of the variables to determine the explanatory variable on the growth of the plant.
Hence, the control variable that can be used in the research experiment is to ensure that the two set of plants will receive the same amount of water and light.
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