intra-EUTRA S1切换流程(含切换准备过程)中的四条消息依次为( )。
A. HANDOVER REQUIRED、HANDOVER REQUEST、HANDOVER COMMAND、HANDOVER REQUEST ACKNOWLEDGE
B. HANDOVER REQUEST、HANDOVER REQUIRED、HANDOVER COMMAND、HANDOVER REQUEST ACKNOWLEDGE
C. HANDOVER REQUEST、HANDOVER REQUIRED、HANDOVER REQUEST ACKNOWLEDGE、HANDOVER COMMAND
D. HANDOVER REQUIRED、HANDOVER REQUEST、HANDOVER REQUEST ACKNOWLEDGE、HANDOVER COMMAND