With circuit switching a __(1)__path is established between two stations for communication . Switching and transmission resources within the network are __(2)__for the exclusive use of  the circuit for the duration of the connection . The connection is__(3)__：Once it is established ，It  appears to attached devises as if there were a direct connection. Packet switching was  designed to  provide a more efficient faciltiy than circult switching for (   ) data tratffic.  Each packet contains some portion of the user data plus control information needed for proper functioning of the network. A  key distinguishing element of packet_switching networks is  whether the internal operation is datagram or virtual circuit . With internal virtual circuits,a route is defined between two endpoints and all packets for  that virtual circuit follow the__(4)__ route, With internal datagrams,each packet is treated independently,and packets intended for the same destination may follow different routes.

（1）A．unique

B. dedicated

C. nondedicated

D.independent

（2）A．discarded

B. abandoned

C. reserved

D. broken

（3）A． indistinct

B.direct

C.indirect

D. transparent

（4）A．casual

B.bursty

C.limited

D.abundant

（5）A．same

B. different

C. single

D.multiple

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