Which of the following is an example of a paired design? select one: a. a teacher compares the scores of students in her class on a standardized test with the national average score. b. a teacher compares the pretest and posttest scores of a group of students. c. a teacher calculates the average scores for students on a pair of tests and tests to see if this average is significantly larger than 80%. d. a teacher compares the scores of students using a computer-based method of instruction with the scores of other students using a traditional method of instruction.