题目内容：
根据下面的文字资料回答题Frame Relay is simplified form of（ 1 ），similar in principle to（ 2 ），in which synchronous，framesof data are routed to different destinations depending on header information．Packets are routed throught one or moreVirtual Circuits known as（ 3 ）．Most Virtual Circuits are（ 4 ）；which means that thenetwork provider sets up all DLCI connections at subscription time．（ 5 ）are also part of the Frame Relayspecification．They privide a link that only lastsonly as long as the session．文中（ 1 ）处正确的答案是（ ）。
A．Datagram Switching B．Datagram Routing
C．Packet Switching
D．packet Routing
参考答案：
答案解析：