Experimental Design: Plant Growth Study

In a study investigating the effect of different light wavelengths on plant growth, researchers designed an experiment with three groups of plants exposed to red, blue, and no light. Each group consisted of ten plants, and the height of each plant was measured after four weeks. The researchers hypothesized that plants exposed to blue light would grow taller than those exposed to red light, while plants in darkness would exhibit the least growth.

After conducting the experiment, the researchers recorded the average height of the plants in each group as follows: red light group averaged 15 cm, blue light group averaged 20 cm, and the no light group averaged 5 cm. To ensure the validity of their findings, the researchers also controlled for other variables such as soil type, water availability, and initial plant height.

Based on this experimental design, which statement is true about the experimental approach used in this study?