H.248协议中以下哪个是MG发给MGC的心跳消息?()
A.!/1[192.168.6.71]:2944Transaction=5{Context=-{Auditcapability=A0{Audit{Media,packages}}}}
B.!/1[192.168.6.71]:2944Transaction=20{Context=-{AuditValue=ROOT{Audit{}}}}
C.!/1[191.2.6.213]:2944Transaction=275{Context=-{Notify=ROOT{ObservedEvents=0{19700101T011
D.!/1[192.168.1.119]:2944Transaction=2050{Context=-{AuditValue=ROOT{Audit{Packages}}}}