单项选择题

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