When does the state of an OSPF adjacency change from $$loading$$ to $$full$$ on a router?a$.$ When the router receives a hello packet from its neighbor that contains its own router ID in the neighbor list.b$.$ When the router receives a database description packet from its neighbor.c$.$ When the router sends a link state request packet to its neighbor, asking for detailed link state information.d$.$ When the router and its neighbor exchange link state request, link state update, and link state ack packets as needed until their link state databases are synchronized.