4. In the following network, B1-B4 are all bridges, and the number beside each port of a bridge is the cost of transmitting a frame from the bridge onto the LAN directly connected by the port. B1 is selected as the root bridge. Answer the following questions related to the spanning tree algorithm: (a) In the following diagram, draw the minimum cost path from the root bridge to each LAN and bridge. (b) In the following diagram, mark the "root port" of each bridge by placing a letter " R " beside the port. (c) What is a "designated bridge" of a LAN? (d) In the above diagram, place a letter " D " beside the port of the designated bridge of each LAN. (e) In the above diagram, mark an " x " on the ports that are in "blocking state" after running the spanning tree algorithm