A Nurse Has Applied for a Job and During the Interview

A nurse has applied for a job and during the interview was told that the health care facility is a "closed shop." What does that phrase mean to the nurse?

A) The nurse must join a union to be employed there.
B) The nurse can join the union if desired, but it is not required.
C) This facility is closed to all union activities.
D) The facility is negotiating with several unions for control of the employees.