Before communication, the exchange mode that needs to establish physical connection between the sender and the receiver is（）
A. Circuit switching
B. Message exchange
C. Packet switching
D. Store and forward exchange
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Among the following factors, the channel data transmission rate will not be affected（）
A. Signal propagation speed
B. Signal to noise ratio
C. Bandwidth
D. Modulation rate
Compared with multimode fiber, the main characteristics of single-mode fiber are（）
A. Long transmission distance, high cost and thin core
B. Long transmission distance, high cost and thick core
C. Short transmission distance, low cost and thick core
D. Short transmission distance, low cost and thin core
One of the following network devices that works at the physical layer is（）
A. Hub
Bridge
C. Router
D. Ethernet Switch
Baud rate means（）
A. Number of transmitted signal symbols per second
Bits transmitted per second
C. Number of possible signal changes per second
D. Bytes transferred per second
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