(41) is a protocol that a host uses to inform a router when it joins or leaves an Internet multicast group.
(42) is an error detection code that most data communication networks use.
(43) is an interior gateway protocol that uses a distance vector algorithm to propagate routing information.
(44) is a transfer mode in which all types of information are organized into fixed form cells on an asynchronous or non-periodic basis over a range of media.
(45) is an identifier of a web page.

A．ICMP

B．SMTP

C．IGMP

D．ARP