An educational psychologist studies the effect of frequent testing on retention of class material.The students are randomly assigned to one of two sections of an introductory course.In one section of an introductory course, students are given quizzes each week.A second section of the same course receives only two tests during the semester.At the end of the semester, both sections receive the same final exam. How many samples are in this research?

a. 0 b. 3 c. 2 d. 1