Which of these statements about PIM join messages in classic PIM-SM is true?
A . PIM join messages are sent every 60 seconds to refresh the upstream router’s mroute state for the multicast tree.
B . Routers send a PIM join acknowledgement in response to each PIM join message received from a downstream router.
C . PIM join messages are only sent when the multicast distribution tree is being established.
D . PIM join messages are sent every three minutes to refresh the upstream router’s mroute state for the multicast tree.
Answer: A