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, frames of data are routed to different destinations depending on header reformation .Packets are routed throught one or more Virtual Circuits known as（ 3 ） . Most Virtual Circuits are（ 4 ）,which means that the network provider sets up all DLCI connections at subscription line.（ 5 ）.arc also part of the Frame Relay specification . They privide a link that only lasts only 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
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答案解析：