单项选择题
A.A
B.B
C.C
D.D
单项选择题 Which command displays the results of a loopback test you executed on a CT3 line module in slot 5, port 0, t1 number 4?（）
单项选择题 Which command is used on the local router to invoke a loopback mode on a remote ERX Edge Router?（）
单项选择题 Which command prompts an ERX Edge Router to go into a loopback mode on T1 number 1 when a command is received from a remote ERX Edge Router?（）
