A$/$an $\_\_\_\_\_\_\_\_\_\_$ is a trade union security agreement where the employer hires union and non$-$union workers. Employees are not forced to join a trade union to remain employed. However, employees who are not members of a trade union must pay a fee to cover the cost of collective bargaining.

a$.$

closed shop agreement

b$.$

agency shop agreement

c$.$

open$-$system approach

d$.$

closed$-$system approach