What Is the Purpose of a 'Reopener Clause

What is the purpose of a 'reopener clause'?

A) To give notice to management that the union wants to make changes
B) To allow the parties on mutual consent to revise any part of the collective agreement
C) To allow the parties to renegotiate terms of the collective agreement while it is still in effect
D) To give notice to the union that management wants to make changes