Which of these statements about PIM join messages in classic PIM-SM is correct？（）

• 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 first being established.
• D、PIM join messages are sent every three minutes to refresh the upstream router’s mroute state for the multicast tree.