Which command(s) will enable you to configure only serial interface 0 on a Cisco router?

A.
router>interface serial 0

B.
router#interface serial 0

C.
router(config)#interface serial 0

D.
router(config-if)#interface serial 0

Explanation:
You can use either the router(config)# interface serial 0 command or the router(config-if)# interface serial 0
command to configure serial interface 0 on the router. To perform configuration changes on a single interface,
you must either enter interface configuration mode for that interface, or simply execute the command to enter
configuration mode for another interface while still at the configuration prompt for the previous interface.
Router configuration mode (as indicated by the prompt router(config)#) allows global configuration of the router.
This mode, also referred to as the global configuration mode, must be entered as a precursor to entering the
interface configuration mode for a specific interface. The sequence of commands and prompts to arrive at this
mode would be:
Router> enable (enters privileged mode)
Router# config t (enters global configuration mode, t is short for terminal)Router(config)# interface serial 0 (enters interface configuration mode for the serial 0 interface)
Router(config-if)#
At this point, any commands executed would be configuration changes limited to the serial 0 interface. For
example, to place an address on the interface, enable the interface, and save the configuration, the command
series and prompts would be:
Router> enable
Router# config t
Router(config)# interface serial 0
Router(config-if)# ip address 192.168.20.1 255.255.255.0 (addresses the interface)
Router(config-if)# no shutdown (enables or “turns on” the interface)
Router(config-if)# exit (exits global configuration mode)
Router(config)# exit (exits privileged mode)
Router# copy running-config startup config (copies the changes to the configuration file on the router)
Alternately, you could enter interface configuration mode for one interface while still in configuration mode for
another interface, as shown below. After entering the interface serial 1 command, you will be editing serial 1
instead of serial 0.
Router(config)# interface serial 0
Router(config)#
Router(config)# interface serial 1
You should not use the command router> interface serial 0. User EXEC mode, as indicated by the prompt
router>, provides limited access to a router and is the initial mode you see after authenticating to the router. The
subcommand interface serial 0 is not functional before you proceed to global configuration mode and interface
configuration mode for a specific interface.
You should not use the command router# interface serial 0. Privileged mode (as indicated by the prompt
router#) must be traversed to get to global configuration mode before you can execute the subcommand
interface serial 0. This subcommand is not functional while you are still in privileged mode.
Objective:
Infrastructure Management
Sub-Objective:
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