Which of the protocols below, operates at Layer 2 of the OSI model, and is used to maintain a loop-free network?（）

A. Layer 2
B. Layer 3
C. Layer 4
D. Layer 5
E. Layer 6
F. Layer 7

A.ARP
B.RARP
C.TCP
D.UDP
E.IP
F.ICMP
G.BootP

A. Two switches are connected via a trunk. Both switches have been assigned an IP address, subnet mask, and default gateway. A ping is successful between the two switches. However, VLAN information is not passed from one switch to the other switch.
B. A router and a switch have been assigned an IP address, a subnet mask, and a default gateway. They are directly connected, but a ping between the two devices fails.
C. A router connects to another router through a serial interface. Both routers have been assigned an IP address, subnet mask, and default gateway. The routers are running RIP and the networks directly connected to the first router do not pass to the second router. However, a ping from one router to the serial interface of the directly connected neighboring router is successful.
D. A router connects to another router via a switch. Both routers and the switch have been assigned an IP address, subnet mask, and default gateway. Neither the routers nor the switch are able to ping one of the hosts that is directly to the switch.