A student carries out an experiment to investigate how concentration affects the rate of diffusion using methylene blue dye in agar jelly. The student adds different concentrations of methylene blue dye to five wells in an agar plate. The diagram shows the experimental setup: 
The student performs the experiment using three separate agar plates, measuring the diameter of the stained circle in centimeters. The results are shown in the table below:
| Concentration of methylene blue (%) | Diameter of circle in plate 1 (cm) | Diameter of circle in plate 2 (cm) | Diameter of circle in plate 3 (cm) | Average (mean) diameter (cm) |
|---|---|---|---|---|
| 1.0 | 0.8 | 0.9 | 0.7 | 0.8 |
| 2.0 | 1.4 | 1.3 | 1.5 | 1.4 |
| 3.0 | 1.8 | 2.0 | 1.9 | ? |
| 4.0 | 2.2 | 2.1 | 2.3 | 2.2 |
| 5.0 | 2.3 | 2.4 | 2.2 | 2.3 |
Describe what is meant by the term diffusion.
Calculate the average diameter for the 3.0% concentration of methylene blue dye.
Describe the effect of methylene blue concentration on diffusion.
Explain the relationship between dye concentration and diffusion.
State two variables that the student should control in this experiment to ensure that the results are valid.