What is the Cisco IOS default behavior for switching from the shared tree to the shortest path tree in PIM-SM operations？（）
- Aimmediately after receiving the first packet on the shared tree for a given （S，G）
- Bafter receiving over 1 kb/s traffic onthe shared tree for a given （S，G）
- C10 seconds after receiving the first packet on the shared tree for a given （S，G）
- D30 seconds after receiving the first packet on the shared tree for a given （S，G）
- Eafter receiving over 10 kb/s traffic onthe shared tree for a given （S，G）