Which statement describes optional transitive BGP attributes?

A.
They must be supported in all BGP implementations, but do not have to be included in every BGP update.

B.
If they are not recognized, they are ignored and not passed to other peers.

C.
They must be supported by all BGP implementations and must be included in every BGP update.

D.
Although not required, they should be passed along, unchanged to other BGP peers when included.