A Researcher Is Interested in Whether Participants in a Drug

A researcher is interested in whether participants in a drug treatment program are successful in their rehabilitation. He asks for volunteers to participate in a long-term study of post-treatment success. Twenty of the 130 participants volunteer to participate in the four-year follow up study. The bias in this sampling plan is best described as:

A) random sampling error.
B) selection bias.
C) discrimination.
D) error.