A closed shop

A) is an arrangement unions have with employers limiting employment to union members only.
B) is one in which union workers cannot be hired.
C) allows a firm to close its doors at will.
D) is legal in the United States.

A closed shop is a workplace where only employees who are already members of a particular union are allowed to work. This arrangement is made through collective bargaining agreements between unions and employers. Therefore, option A is correct. Option B is incorrect because union workers are the only ones who can be hired in a closed shop. Option C is also incorrect as it refers to a firm's ability to shut down its operations, which is not related to closed shops. Finally, option D is incorrect because closed shops were outlawed by the Taft-Hartley Act of 1947 in the United States, although there are still certain exceptions and variations allowed in some states.