Which of the Following Is an Unbiased Research Sample

Multiple Choice

Which of the following is an unbiased research sample?

A) a questionnaire emailed to all 18 clients of a social worker has 16 returned completed
B) a survey on self-care posted on a website for cancer patients gets 1,860 responses
C) 20 first-year students are randomly selected and agree to interviews about college transition
D) every student in a class completes a survey about their family's socio-economic status