$($Learning Outcome $11)$ The labor agreement stipulating that employers, although free to hire whomever they choose, may retain only union members and that all employees hired into positions covered under the terms of a collective bargaining agreement must, after a specified probationary period of typically thirty to sixty days, join the union or forfeit their jobs is known as a:

Question $34$ options:

union shop.

maintenance of membership.

closed shop.

agency shop.

open shop.