One problem with mail surveys is non‑response bias. This means a. usually only a small percentage of the surveys are returned. b. many questionnaires will be returned with missing answers.  c....

One problem with mail surveys is non‑response bias. This means

a. usually only a small percentage of the surveys are returned.

b. many questionnaires will be returned with missing answers.

 c. individuals who return surveys may not be representative of the general population.

 d. attempts to increase the number of people who return the survey may produce a biased sample.