A.It divides the network communication process into smaller and simpler components， thus aiding component development， design， and troubleshooting.
B.It enables equipment from different vendors to use the same electronic components， thus saving research and development funds.
C.It supports the evolution of multiple competing standards， and thus provides business opportunities for equipment manufacturers.
D.It encourages industry standardization by defining what functions occur at each layer of the model.
E.It provides a means by which changes in functionality in one layer require changes in other layers.

A.FTP
B.Telnet
C.SMTP
D.DNS
E.HTTP
F.POP3