"Mandatory" Subjects of Bargaining

"Mandatory" subjects of bargaining:

A) must be negotiated if one party raises the issue.
B) must be negotiated until agreement is reached if one party raises the issue.
C) do not have to be negotiated if one party raises the issue.
D) mandates that a federal mediator will resolve the differences between the parties if they can't reach agreement on one or more bargaining subjects.
E) mandates that a federal mediator must approve the agreement.