Which of the Following Is True About a Closed Shop

Which of the following is true about a closed shop?

A) It requires that any bargaining-unit employee who was hired after a date specified in the agreement must become a union member within a specific time as a condition of continued employment.
B) It requires that any bargaining-unit employee employed with the firm for a specific time must become a union member as a condition of continued employment.
C) It requires that employers hire only union members.
D) It requires that any bargaining-unit employee who is not a union member must pay a service fee to the union for its representation activities.