At Murphy Elementary School, the average distance all students live from the school is 6.4 miles. Jason, Sam, Clint, and Cori each survey a different sample of 20 students from Murphy Elementary School in order to find out how far these students live from the school. Their sample averages, respectively, are 9.7 miles, 12.2 miles, 10.4 miles, and 7.8 miles. The fact that these sample averages are all different illustrates the concept of ___________.

A. categorization B. nonresponse C. stratification D. bias E. variability