Which of the following is an example of a paired design?
A. A teacher compares the pretest and posttest scores of a group of students.
B. 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.

C. A teacher compares the scores of students in her class on a standardized test with the national average score.
D. 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%.