问题：

[单选] 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.