Robert is gathering data for his graduate research project. He targets psychology students as his participants. He randomly selects several psychology classes of which to administer his questionnaire. Which of the following is true about Robert's participants?

a. The students in the psychology classes represent the population and all the psychology students on campus also represent the population.
b. The students in the psychology classes represent the population and all the psychology students on campus represent the sample.
c. The students in the psychology classes represent the sample and all the psychology students on campus also represent the sample.