Each person in a simple random sample of 1,800 received a survey, and 267 people returned their survey. How could nonresponse cause the results of the survey to be biased?

a. Those who did not respond reduced the sample size, and small samples have more bias than large samples.

b. Those who did not respond caused a violation of the assumption of independence.

c. Those who did not respond are indistinguishable from those who did not receive the survey.

d. Those who did not respond may differ in some important way from those who did respond.

e. Those who did not respond represent a stratum, changing the random sample into a stratified random sample.

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