In BGP routing，what does the rule of synchronization mean？（）

A.A BGP router can only advertise an EBGP learned route，provided that the route is an IGP route in the routing table.
B.A BGP router can only advertise an IBGP learned route，provided that the route is an IGP route in the routing table.
C.A BGP router can only advertise an IBGP learned route，provided that the route is an IGP route that is not in the routing table.
D.A BGP router can only advertise an EBGP learned route，provided that the route is a metric of 0 in the BGP table.