Zach is conducting an experiment to observe which type of fertilizer will allow a plant to grow the tallest. He puts three plants in identical pots and places them in the same location. He waters the plants each day with the same amount of water. Then, Zach measures the plant growth and records his observations. Which of these is the control group in Zach’s experiment?

a) the growth rate of each plant
b) the pot that does not contain fertilizer
c) the type of fertilizer used in each plant
d) the amount of water given to each plant