题目内容：
work（ ）,systems that effectively isolate an organization's internal network structure from an （请作答此空） network, such as the INTERNET is becoming increasingly popular. These firewall systems typically act as application-layer（ ）between networks, usually offering controlled TELNET, FTP, and SMTP access. With the emergence of more sophisticated（ ）layer protocols designed to facilitate global information discovery , there exists a need to provide a general （ ）for these protocols to transparently and securely traverse a firewall.
A.exterior
B.Internal
C.center
D.middle
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答案解析：