【单选题】
通过Um接口监测仪表观察,可能观察到的GPRS ATTACH信令流先后顺序是()
A、ATTCHREQUEST-SECURITYFUNCTION-ATTCHACCEPT-ATTCHCOMPLETE
B、ATTACHREQUEST-SECURITYFUNCTION-ATTACHCOMPLETE-ATTACHACCEPT
C、ATTACHREQUEST-ATTACHACCEPT-SECURITYFUNCTION-ATTACHCOMPLETE
D、SECURITYFUNCTION-ATTACHREQUEST-ATTACHACCEPT-ATTACH