Which of the Following Is True About a Union Shop

Which of the following is true about a union shop agreement?

A) A union shop agreement leaves the union membership to the employee's discretion.
B) This is unlawful because it requires an employer to discriminate.
C) An employer entering into this agreement may hire a nonunion employee,but all employees who are hired must then become union members.
D) This requires employees who are members of the union before being hired to remain as members once they are hired,unless they take advantage of an "escape clause."
E) An employer who accepts this agreement decides to hire only union members as long as the union can supply him with a sufficient number of qualified workers.