A music professor offers his 40 students the option of coming to an additional rehearsal session the week before their juries (musical final exams

A music professor offers his 40 students the option of coming to an additional rehearsal session the week before their juries (musical final exams.) In order to decide whether these extra sessions actually help students, he keeps track of who attends them and compares their jury scores to those of students who did not schedule extra sessions.

This study is a(n):

A) matched pairs design.

B) randomized block design.

C) nonrandomized experiment.

D) observational study.

E) completely randomized experiment