Andy conducts a study to improve reading scores in elementary schools. He obtains a sample of second-graders whose reading level was very much below average. He enrolls these students in a reading program and then measures their reading level six months later. Reading levels were significantly higher after the program than before. Because Andy selected participants on the basis of their extreme scores, he should be concerned about:a. maturation.b. statistical regression.c. history.d. instrumentation.