The Key Difference Between an Agency Shop and an Open

The key difference between an agency shop and an open shop is that in an agency shop:

A) workers must join the union within a stipulated time period (usually 30,60,or 90 days) in order to keep their jobs,but in an open shop the workers are not required to join the union.
B) the union is restricted to a limited number of employees who perform specific types of jobs,but in an open shop membership in the union is available to all workers.
C) workers who do not join the union must pay a fee or regular dues,while in an open shop workers who choose not to join the union do not have to pay any union fees or dues.
D) workers must agree not to join a union in order to keep their jobs while in an open shop workers are free to join a union if they wish,but they are not required to do so.