In the network shown in the figure, the interconnection lines between RT1, RT2, RT3, RT4 and RT5 are all Gigabit Ethernet lines, running OSPF, and the corresponding network segments are released to achieve routing intercommunication. The routers all run the PIM SM protocol, and the RP is 1.1.1.1. RT4 is connected to a receiver, and IGMP is enabled on the corresponding interface. When the multicast traffic initially forwarded along the RPT is switched to the SPT, which of the following routers constitute the SPT? (Multiple choice)