Identifying Flaws in Experimental Design

A team of researchers is conducting an experiment to test the effects of different fertilizers on the growth rate of tomato plants. They set up three groups of tomato plants: Group A receives Fertilizer X, Group B receives Fertilizer Y, and Group C receives no fertilizer (control group). Each group contains 10 plants, all of the same species, size, and starting health. The researchers measure the height of the plants every week for eight weeks.

Based on this experimental design, which of the following statements most accurately describes a potential flaw in the experiment?