What is the grace period in a graceful restart situation?

A.
how long the supervisor waits for NSF replies

B.
how often graceful restart messages are sent after a switchover

C.
how long NSF-aware neighbors should wait after a graceful restart has started before tearing
down adjacencies

D.
how long the NSF-capable switches should wait after detecting that a graceful restart has
started, before verifying that adjacencies are still valid