简述侦察兵作用。
In BGP routing， what does the rule of synchronization mean？（） ["A BGP router can only advertise an EBGP learned route， provided that the route is an IGP route in the routing table.","A BGP router can only advertise an IBGP learned route， provided that the route is an IGP route in the routing table.","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.","A BGP router can only advertise an EBGP learned route， provided that the route is a metric of 0 in the BGP table."]
焦虑
《全国铁路通用乘车证》是铁路总公司根据国家公安、安全、司法和机要部门的特殊工作性质（）的特殊乘车证。 ["发售","印制","签发","专控"]
下列指数中，（）是质量指标指数。 ["单位成本指数","工人人数指数","销售量指数","产品产量指数"]
《中华人民共和国教育法》规定，“维护受教育者、（）”是学校应当履行的义务之一。
