A.Log
B.Log-input
C.Log-output
D.Logging

A.Port 3/12 was already up and forwarding before the loop was created.
B.Port priority is based on lowest priority and lowest port number.
C.You connected the wire on port 3/18 last.
D.None of the above， it is purely random.

A.Only use fiber in pairs.
B.Configure root guard on your switches.
C.Do not use fiber but use copper.
D.Configure UDLD to prevent one-way link conditions.

A.origin，AS-path，next-hop
B.AS-path，origin，MED
C.AS-path，origin，weight
D.AS-path，weight，MED

A.O IA
B.O E1
C.O E2
D.O

A.source IP address and the protocol （such as SSL，HTTPS，SSH）
B.the packet Time to Live and the source IP address
C.type of service header and DSCP value
D.destination IP address

A.An update packet with init flag set from a known， already established neighbor relationship was received by one of the routers.
B.The ARP cache was cleared.
C.The counters were cleared.
D.The IP EIGRP neighbor relationship was cleared manually.