Which of the following statements about the PIM SM registration process are correct?

Which of the following statements about the PIM SM registration process are correct?
A . Unlike the PUSH method of PIM DM, in PIM SM, the multicast source starts sending multicast traffic only when there are downstream receivers.
B . When the multicast source initially sends traffic, the DR connected to it will encapsulate several empty Register messages to register with the RP
C . After receiving the source registration message for the first time, RP will send a (*, G) join message to the multicast source and establish a (*, G) table entry along the way.
D . After the RP receives the multicast forwarding traffic from the multicast source, it forwards the traffic to its downstream interface until it reaches the multicast receiver, forming an RPT from the RP to the multicast receiver along the way.

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Answer: D