单项选择题

External Handover过程按时间顺序先后的正确信令流程为()

A.HO REQUI RED→HO REQUEST→HO REQUEST ACK→HO COMMAND
B.HO REQUEST→HO REQUEST ACK→HO REQUIRED→HO COMMAND
C.HO REQUEST→HO REQUI RED→HO COMMAND→HO REQUEST ACK
D.HO REQUI RED→HO COMMAND→HO REQUEST→HO REQUEST ACK