“A student performed an investigation in which two ivy plants were planted in two separate containers. One of the containers had earthworms mixed in with the soil,
and the other container had soil and no earthworms. The plants were given the
same amount of water and exposed to the same amount of sunlight. The student
observed that after several weeks of growth, the plant exposed to earthworms
appeared to be healthier and exhibited more growth. To conclude that the
relationship between the plant and the earthworm is an example of mutualism, the
student must perform which of the following follow-up investigations?”