A scientist performs an experiment to see how plants respond to acid rain. He places the water plants in water of varying pH measurements. The pH levels are 2.5, 6.0, 7.0, and 9.0. Each day, he measures the health of the plants by counting the number of leaves on the plant. The plants used in this experiment typically grow in water at a neutral pH. Why was only one variable tested in this experiment?

The scientist tested one variable to determine what caused the number of leaves to change.
It just happened that way; you can test as many variables as you want.
The scientist ran out of materials so he tested only one variable.
The scientist tested two variables.