In an experiment to test how therapies A and B improve the eyesight of rats, a scientist groups 20 rats based on their vision. The four rats with the worst vision are in one group, the next four rats with second-to-worst vision are in another group, and so on until the final group has the rats with the best vision. From each group, two rats are randomly selected to receive therapy A, while the remaining two rats in each group receive therapy B. What are the experimental units in this experiment?
1) The experimental units are therapies A and B.
2) The treatment is the eyesight improved.
3) This is an example of a matched pairs experiment.
4) The experimental units are the 20 rats.
5) This is an example of a block design.
6) This is an example of a completely randomized experiment.
7) The treatments are therapies A and B.