A hospital did a ten-year study on patients who used its services. One thousand patients were randomly selected and followed over a period of ten years to determine the average number of visits per year. By the end of the ten year period some of the patients had left the area. Identify the problem with the poll. A. Some of the sample data are missing.

B. A nonresponse occurred.

C. The relationship between two variables implies one causes the other.

D. The conclusion is based on a voluntary response sample.