A teacher wants to investigate whether listening to music while taking a math test increases scores. He instructs half of his 36 students to listen to music while taking a test and the other half to not listen to music during the test. To
determine who listens and who does not, the teacher identifies the two students who scored highest on the last test
and randomly instructs one to listen to music and one to not. The teacher does the same for the next two highest-
scoring students and continues in this manner until each student has their instructions. Which of the following best
describes this plan?
This is a cluster sample.
This is an observational study.
This is a stratified random sample.
This is an experiment with blocking.
O This is a completely randomized experiment.