( ) is called the gateway of network security, which is used toidentify what kind of data packets

(   ) is called the gateway of network security, which is used toidentify what kind of data packets can enter the enterprise intranet. （2020年系统集成项目管理工程师下半年）
 A. Anti-virus software
 B. Trojan horse
 C. Secret key
 D. Firewall

参考答案： D
参考解析：