To investigate if standing up while studying affects performance in an algebra class, a teacher assigns half of the 30 students in his class to stand up while studying and assigns the other half to not stand up while studying. To determine who receives which treatment, the teacher identifies the two students who did best on the last exam and randomly assigns one to stand and one to not stand. The teacher does the same for the next two highest-scoring students and continues in this manner until each student is assigned a treatment. Which of the following best describes this plan?

a. This is an observational study.

b. This is an experiment with blocking.

c. This is a completely randomized experiment.

d. This is a stratified random sample.

e. This is a cluster sample.