A. After the formation of the shortest-path tree, a join message is sent from R2 to R1.
B. After the formation of the shortest-path tree, a join message is sent from R1 to R5
C. After the formation of the shortest-path tree, a prune message is sent from R1 to R2.
D. After the formation of the shortest-path free, a prune message is sent from R1 to R5.