单项选择题 The Testking Central and Remote offices are configured as shown below: The remote router can be successfully pinged from the central office but the remote users cant access the server at the central office.Based on the output above, what do you suspect is the cause of this problem?（）

单项选择题 While troubleshooting a serial line problem, you enable ppp authentication debugging as shown below: Based on the command output above, what type of handshake was used for PPP authentication?（）

单项选择题 After attempting to telnet into a router, you are denied and you receive the error message "password required, but none set." What configuration changes will allow telnet access into this router?（）