【单选题】

Frame Relay is simplified form of (71) , similar in principle to (72) , in which synchronous, frames of data are routed to different destinations depending on header information.

Packets are routed throught one or more VirtualCircuits known as (73) Most VirtualCircuits are (74) , which means that the network provider sets up allDLCI connections at subscription time. (75) are also part of the Frame Relay specification. They privide a link that only lasts only as long as the session.
A．Datagram Switching
B．Datagram Routing
C．Packet Switchig
D．packet Routing