Jose believes that music helps plants growth faster. So he is designing an experiment to determine how music affects a plants growth. He plans to carry out his experiment by using six different flowering plants (a daisy, a petunia, a geranium, a snapdragon, a begonia, and a violet). Each plant will be placed in one liter of potting soil, and given the same amount of water and sunlight each day. Each plant will be exposed to six types of music for the duration of the experiment. He presented his plan to his teacher for approval, but Mr.Johnson said that he should reevaluate his variables so that his experiment will be reliable. What error did Mr. Johnson find in Jose’s plan?