单选题：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

答案：A