Aprofessor conducts an experiment that investigates the effects of a new statistics cd on student understanding. his one statistics class is used for the sample. he divides the roster into two halves and uses each half as a group. one group receives the cd and the other does not receive the cd. each group consists of 10 students. observations of student classroom engagement are rated by the professor, according to an engagement scale. how could this study be improved? a. randomly assign the students to the two different groups. b. repeat the study again, when a larger sample size can be derived. c. use an instrument that measures student understanding. d. ask another professor to make and rate observations, as well.