A professor is interested in how the 500 students in his class will rate today's lecture. He selects the first 20 students on his class list, reads the names at the beginning of the lecture, and asks them to go online to the course Web site and rate the lecture on a scale of 0 to 5. Which of the following is the most important reason why a confidence interval for the mean rating by all his students based on these data is of little use?
A. The number of students selected is small, so the margin of error will be large.
B. Most of the students selected will not respond.
C. The students selected can't be consideed a random sample from the population of all students in the course.