Under the centralized multi-node network conditions (center equipment U1981), A for branch node 1 POTS user (branch node device is U1911), B and D are SIP user of another branch node 2 (branch node device U1960), C is PSTN user, A register unconditional forwarding service, if a user calls A, will be forwarded to the users B, C now call D through branch node 2 PRA trunk incoming, D transfer the call to A, and then A forwarded to B, B,C call, then at this time the RTP stream case is ( ).

Under the centralized multi-node network conditions (center equipment U1981), A for branch node 1 POTS user (branch node device is U1911), B and D are SIP user of another branch node 2 (branch node device U1960), C is PSTN user, A register unconditional forwarding service, if a user calls A, will be forwarded to the users B, C now call D through branch node 2 PRA trunk incoming, D transfer the call to A, and then A forwarded to B, B,C call, then at this time the RTP stream case is ( ).
A . B<->U1981<->U1911
B . B<->U1960<->U1981<->U1911
C . B<->U1911
D . B<->U1960
E . B<->D<->U1960

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Answer: D