A Professor Believes That Students Who Smoke Cigarettes Tend to Have

Multiple Choice

A professor believes that students who smoke cigarettes tend to have lower grades. He collects data from 1326 randomly selected students at his university and discovers that, on average, students who smoke cigarettes do indeed tend to have lower grade point averages than students who do not smoke.
This study was based on

A) a randomized comparative experiment.
B) a matched pairs experiment.
C) a voluntary response sample.
D) a probability sample.