Packet switching is a method of slicing (71) messages into parcels called“packets，”sending the packets along different communication (72) as they become available，and thenreassembling the packets (73) they arrive at their destination．Prior to the development ofpacket switching，early computer networks used leased，dedicated telephone circuits to (74) with terminals and other computers．In circuit—switched networks such as the telephone system，a complete point—to—point (75) is put together,and then communication can proceed．
A．sequence
B．line
C．board
D．circuit
