Is the Sample Bias or Unbiased

Short Answer

Is the sample bias or unbiased?
a. A professor studying the attitudes of all sophomores at a college asks those who are enrolled in her psychology class. ________
b. A researcher doing a citywide survey on monthly spending at local grocery stores. ________
c. A magazine editor studying the opinion on a gun-control measure from all rifle owners, mails a questionnaire to only those who subscribe to her magazine. ________