As an HR Professional You Should Be Familiar with the Terminology

As an HR Professional you should be familiar with the terminology, practices, and rules governing unions and management in the bargaining process. What is the zipper clause in regard to negotiations?

A) Items in a management-union contract can be "zipped" open and closed as often as necessary.
B) Management is locked out of union meetings.
C) Management and union representations are locked out of union employee meetings.
D) Items in a management-union contract are "zipped" closed, once the agreement is signed by both parties.