Eddie is performing an experiment. he asked the question what type of soil makes plants grow the fastest. he has four plants. he gives plant #1 with potting soil, plant #2 with sand, plant #3 with dirt from his yard, and plant #4 with gravel. all of the plants are kept on the same shelf next to the window and receive the same amount of water. after 4 months, plant #1 is 100 cm tall, plant #2 & #3 are 115 cm tall, and plant #4 is 12 cm tall. what is the manipulated variable in this experiment? what is the responding variable? me!