To test how fertilizer affects aquatic animals, a student prepared four identical containers with water fleas. The student then added different amounts of fertilizer mixed with distilled water to three of the containers and distilled water alone to one of the containers. What is the experimental group in this experiment?

A. All the water fleas in the container with the most fertilizer
B. The water fleas in the container with only distilled water
C. All the water fleas in all the containers
D. The water fleas in all the containers receiving fertilizer