Which statement about PIM sparse mode operations is true？（）

• A、It does not support all underlying unicast routing protocols like BGP
• B、Receivers are registered with RP by their first hop router
• C、From the RP， traffic flows down a source tree to each receiver.
• D、It supports shared trees only， and assumes all hosts want the multicast traffic
• E、Receivers are joined to the shared tree by their local designated router