Erik is interviewing participants for his research study. He has a set of questions that he asks every participant, but sometimes asks follow-up questions as needed. What type of interview is he conducting?

A) semi-structured interview
B) self-report questionnaire
C) projective test
D) longitudinal study

Semi-structured interviews involve a set of predetermined questions that are asked to all participants, but also allow for follow-up questions as needed. This allows for some flexibility and personalization in the interview process. The other options listed, self-report questionnaires and projective tests, do not involve direct interaction with a researcher and do not allow for follow-up questions. A longitudinal study involves collecting data from the same participants over a period of time, but does not necessarily involve interviews.