Router R1 has been configured with the commands rout er eigrp 9 and network 172.16.2.0 0.0.0.255, with no other current EIGRP configuration. R1's (working) Fa0/0 interface has been configured with IP address 172.16.2.2/26. R1 has found three EIGRP neighbors reachable via interface Fa0/0, including the rou ter with IP address 172.16.2.20. When the engineer attempts to add the neighbor 172.16.2.20 fa0/0 command in EIGRP configuration mode. Which of the following occurs？（）

A. Fa0/0 fails
B. The command is rejecteD
C. The existing three neighbors fail
D. The neighborship with 172.16.2.20 fails and then reestablishes
E. None of the other answers is correct