【单项选择题】
A TestKing.com technician is troubleshooting connectivity problems between two routers that are directly connected through the serial line. The technician notices that the serial line is up but cannot see any neighbors displayed in the output of the show cdp neighbors command.<br /> In which OSI layer is the problem most likely occurring?（）<p> A TestKing.com technician is troubleshooting connectivity problems between two routers that are directly connected through the serial line. The technician notices that the serial line is up but cannot see any neighbors displayed in the output of the show cdp neighbors command.<br /> In which OSI layer is the problem most likely occurring?（）</p>
A、A. Physical
B、B. Data link
C、C. Network layer
D、D. Transport layer
E、E. Application layer
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