In the Context of a Collective Agreement, a "Union Shop

In the context of a collective agreement, a "union shop" clause

A) is required by law.
B) signifies that the employer is involved in sales, at either the retail or wholesale level.
C) allows a union to "shop around" for new employees.
D) means that a person must join a union in the early stages of their employment.
E) means management cannot hire anyone who is not already a member of the union.